Employee Benefit Plans - Effect of Retrospective Presentation Change Related to the Net Periodic Cost of Defined Benefit Pension Plans on Consolidated Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 440	$ 620	$ 896	$ 1,228
Cost of revenues
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		123,734		242,806
Net periodic benefit cost, effect of change, higher/ (lower)		(112)		(221)
Cost of revenues | Previously reported
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		123,846		243,027
General and administrative expenses
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		24,425		48,462
Net periodic benefit cost, effect of change, higher/ (lower)		(13)		(27)
General and administrative expenses | Previously reported
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		24,438		48,489
Selling and marketing expenses
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		13,095		26,435
Net periodic benefit cost, effect of change, higher/ (lower)		(1)		(2)
Selling and marketing expenses | Previously reported
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		13,096		26,437
Other income, net
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		2,512		5,698
Net periodic benefit cost, effect of change, higher/ (lower)		(126)		(250)
Other income, net | Previously reported
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		$ 2,638		$ 5,948