Significant Accounting Policies - Schedule of impact of Topic 606 adoption on Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Operations data:
Revenues	$ 885,288	$ 855,933
Costs and Expenses:
Direct costs	865,446	853,232
Income tax expense	(2,532)	27,094
Net loss	171,545	$ (120,353)
Balances Without Adoption of ASC 606
Statement of Operations data:
Revenues	878,655
Costs and Expenses:
Direct costs	868,561
Income tax expense	(2,502)
Net loss	161,827
Effect of Change Higher/(Lower) | ASU 2014-09
Statement of Operations data:
Revenues	6,633
Costs and Expenses:
Direct costs	(3,115)
Income tax expense	30
Net loss	$ 9,718