Business segments - Schedule of Segment Reporting Results By Segment (Details) $ in Thousands |
3 Months Ended | 12 Months Ended | |||||||||
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Sep. 30, 2016
USD ($)
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Jun. 30, 2016
USD ($)
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Mar. 31, 2016
USD ($)
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Dec. 31, 2015
USD ($)
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Sep. 30, 2015
USD ($)
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Jun. 30, 2015
USD ($)
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Mar. 31, 2015
USD ($)
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Dec. 31, 2014
USD ($)
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Sep. 30, 2016
USD ($)
segment
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Sep. 30, 2015
USD ($)
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Sep. 30, 2014
USD ($)
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| Segment Reporting [Abstract] | |||||||||||
| Number of business segments | segment | 3 | ||||||||||
| Financial information for each of the Company's business segments | |||||||||||
| Revenue | $ 623,091 | $ 617,094 | $ 606,453 | $ 556,722 | $ 578,683 | $ 572,301 | $ 481,794 | $ 467,043 | $ 2,403,360 | $ 2,099,821 | $ 1,700,912 |
| Gross profit | 152,930 | 151,379 | 147,667 | 110,215 | 125,307 | 143,798 | 124,345 | 119,267 | 562,191 | 512,717 | 452,123 |
| Selling, general and administrative expense | 268,259 | 238,792 | 220,925 | ||||||||
| Operating income | 81,308 | 84,609 | 77,900 | 42,786 | 61,527 | 71,067 | 62,007 | 65,231 | 286,603 | 259,832 | 225,308 |
| Amortization of intangible assets | (13,377) | (9,348) | (5,890) | ||||||||
| Acquisition-related expenses | (832) | (4,745) | 0 | ||||||||
| Gain on sale of a business | $ 6,880 | $ 0 | $ 0 | ||||||||
| Percentage of total revenue | 11.90% | 12.40% | 13.20% | ||||||||
| Depreciation and amortization | $ 58,404 | $ 46,849 | $ 42,778 | ||||||||
| Other/Corporate | |||||||||||
| Financial information for each of the Company's business segments | |||||||||||
| Selling, general and administrative expense | (622) | 2,127 | 0 | 650 | (102) | 8 | 95 | 5 | 2,155 | 6 | 614 |
| Other | (2,155) | (6) | (614) | ||||||||
| Segment Reconciling Items | |||||||||||
| Financial information for each of the Company's business segments | |||||||||||
| Amortization of intangible assets | (3,449) | (3,517) | (3,262) | (3,149) | (3,166) | (3,275) | (1,432) | (1,475) | (13,377) | (9,348) | (5,890) |
| Acquisition-related expenses | (257) | 0 | (529) | (46) | $ 172 | $ 2,459 | $ 1,514 | $ 600 | (832) | (4,745) | 0 |
| Gain on sale of a business | $ 427 | $ 6,453 | $ 0 | $ 0 | 6,880 | 0 | 0 | ||||
| Health Services | |||||||||||
| Financial information for each of the Company's business segments | |||||||||||
| Revenue | 1,298,304 | 1,109,238 | 906,650 | ||||||||
| Gross profit | 292,181 | 254,108 | 213,728 | ||||||||
| Selling, general and administrative expense | 107,155 | 99,815 | 98,172 | ||||||||
| Operating income | $ 185,026 | $ 154,293 | $ 115,556 | ||||||||
| Percentage of total revenue | 14.30% | 13.90% | 12.70% | ||||||||
| Depreciation and amortization | $ 31,916 | $ 27,694 | $ 23,994 | ||||||||
| U.S. Federal Services | |||||||||||
| Financial information for each of the Company's business segments | |||||||||||
| Revenue | 591,728 | 502,484 | 341,840 | ||||||||
| Gross profit | 138,168 | 118,646 | 110,793 | ||||||||
| Selling, general and administrative expense | 74,792 | 59,252 | 47,695 | ||||||||
| Operating income | $ 63,376 | $ 59,394 | $ 63,098 | ||||||||
| Percentage of total revenue | 10.70% | 11.80% | 18.50% | ||||||||
| Depreciation and amortization | $ 9,953 | $ 10,363 | $ 9,557 | ||||||||
| Human Services | |||||||||||
| Financial information for each of the Company's business segments | |||||||||||
| Revenue | 513,328 | 488,099 | 452,422 | ||||||||
| Gross profit | 131,842 | 139,963 | 127,602 | ||||||||
| Selling, general and administrative expense | 84,157 | 79,719 | 74,444 | ||||||||
| Operating income | $ 47,685 | $ 60,244 | $ 53,158 | ||||||||
| Percentage of total revenue | 9.30% | 12.30% | 11.70% | ||||||||
| Depreciation and amortization | $ 16,535 | $ 8,792 | $ 9,227 | ||||||||