Annual report pursuant to Section 13 and 15(d)

Business combinations and disposals - K-12 Education (Details)

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Business combinations and disposals - K-12 Education (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Gain on disposal                 $ 650 $ 6,880 $ 0
Revenue $ 620,903 $ 600,447 $ 622,047 $ 607,564 $ 623,091 $ 617,094 $ 606,453 $ 556,722 2,450,961 2,403,360 2,099,821
Operating income (loss) $ 77,590 $ 81,811 $ 80,335 $ 73,776 $ 81,308 $ 84,609 $ 77,900 $ 42,786 313,512 286,603 259,832
K-12 Education                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Revenue                   2,200 4,700
Operating income (loss)                   (200) $ 900
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Related to Disposition of K-12 Education [Member] | K-12 Education                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Gain on disposal                 $ 700 $ 6,880