Significant Accounting Policies - Schedule of impact of Topic 606 adoption on Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Operations data:
Revenues	$ 787,102	$ 818,601	$ 2,436,514	$ 2,434,687
Costs and Expenses:
Direct costs	677,816	728,743	2,081,125	2,110,901
Income tax expense	3,773	(12,530)	(65,822)	(13,668)
Net income	$ (96,684)	$ (103,490)	298,814	$ (106,745)
Balances Without Adoption of ASC 606
Statement of Operations data:
Revenues			2,433,937
Costs and Expenses:
Direct costs			2,084,654
Income tax expense			(65,786)
Net income			292,743
Effect of Change Higher/(Lower) | ASU 2014-09
Statement of Operations data:
Revenues			2,577
Costs and Expenses:
Direct costs			(3,529)
Income tax expense			(36)
Net income			$ 6,071