Business segments - Schedule of Segment Reporting Results By Segment (Details) $ in Thousands |
3 Months Ended | 12 Months Ended | |||||||||
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Sep. 30, 2018
USD ($)
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Jun. 30, 2018
USD ($)
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Mar. 31, 2018
USD ($)
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Dec. 31, 2017
USD ($)
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Sep. 30, 2017
USD ($)
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Jun. 30, 2017
USD ($)
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Mar. 31, 2017
USD ($)
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Dec. 31, 2016
USD ($)
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Sep. 30, 2018
USD ($)
segment
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Sep. 30, 2017
USD ($)
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Sep. 30, 2016
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 | $ 558,446 | $ 597,855 | $ 612,787 | $ 623,148 | $ 620,903 | $ 600,447 | $ 622,047 | $ 607,564 | $ 2,392,236 | $ 2,450,961 | $ 2,403,360 |
| Gross profit | 138,938 | 154,684 | 148,803 | 151,960 | 162,581 | 152,189 | 152,317 | 144,818 | 594,385 | 611,905 | 562,191 |
| Selling, general and administrative expense | 285,241 | 284,593 | 269,091 | ||||||||
| Operating income | 61,908 | 82,571 | 71,321 | 79,683 | 77,590 | 81,811 | 80,335 | 73,776 | 295,483 | 313,512 | 286,603 |
| Amortization of intangible assets | (10,308) | (12,208) | (13,377) | ||||||||
| Gain on sale of a business | $ 0 | $ 650 | $ 6,880 | ||||||||
| Percentage of total revenue | 12.40% | 12.80% | 11.90% | ||||||||
| Depreciation and amortization | $ 51,884 | $ 55,769 | $ 58,404 | ||||||||
| Corporate/Other | |||||||||||
| Financial information for each of the Company's business segments | |||||||||||
| Selling, general and administrative expense | 865 | 1,032 | 0 | 0 | 1,050 | (90) | 92 | 357 | 2,844 | 1,492 | 2,987 |
| Other | (1,897) | (1,409) | (2,155) | ||||||||
| Segment Reconciling Items | |||||||||||
| Financial information for each of the Company's business segments | |||||||||||
| Amortization of intangible assets | (2,462) | (2,525) | (2,603) | (2,718) | (2,700) | (2,720) | (3,386) | (3,402) | (10,308) | (12,208) | (13,377) |
| Restructuring Costs | (3,353) | (2,242) | 0 | ||||||||
| Acquisition-related expenses | $ (947) | $ 0 | $ 0 | $ 0 | (83) | 0 | 0 | 0 | (947) | (83) | (832) |
| Gain on sale of a business | $ 0 | $ 650 | $ 0 | $ 0 | 0 | 650 | 6,880 | ||||
| Health Services | |||||||||||
| Financial information for each of the Company's business segments | |||||||||||
| Revenue | 1,404,959 | 1,380,151 | 1,298,304 | ||||||||
| Gross profit | 372,628 | 347,325 | 292,181 | ||||||||
| Selling, general and administrative expense | 136,250 | 132,081 | 107,155 | ||||||||
| Operating income | $ 236,378 | $ 215,244 | $ 185,026 | ||||||||
| Percentage of total revenue | 16.80% | 15.60% | 14.30% | ||||||||
| Depreciation and amortization | $ 28,613 | $ 29,114 | $ 31,916 | ||||||||
| U.S. Federal Services | |||||||||||
| Financial information for each of the Company's business segments | |||||||||||
| Revenue | 478,911 | 545,573 | 591,728 | ||||||||
| Gross profit | 126,698 | 139,321 | 138,168 | ||||||||
| Selling, general and administrative expense | 69,312 | 74,345 | 74,792 | ||||||||
| Operating income | $ 57,386 | $ 64,976 | $ 63,376 | ||||||||
| Percentage of total revenue | 12.00% | 11.90% | 10.70% | ||||||||
| Depreciation and amortization | $ 8,478 | $ 11,175 | $ 9,953 | ||||||||
| Human Services | |||||||||||
| Financial information for each of the Company's business segments | |||||||||||
| Revenue | 508,366 | 525,237 | 513,328 | ||||||||
| Gross profit | 95,059 | 125,259 | 131,842 | ||||||||
| Selling, general and administrative expense | 76,835 | 76,675 | 84,157 | ||||||||
| Operating income | $ 18,224 | $ 48,584 | $ 47,685 | ||||||||
| Percentage of total revenue | 3.60% | 9.20% | 9.30% | ||||||||
| Depreciation and amortization | $ 14,793 | $ 15,480 | $ 16,535 | ||||||||