Basis of Condensed Consolidated Financial Statements (Cumulative Effect of Adoption of New Accounting Standard) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net		$ 254,263	$ 255,654	$ 256,595
Prepaid expenses and other current assets		53,515	46,255	47,220
Other assets		119,380	107,428	104,882
Contract liabilities, current		308,659	310,715	303,948
Contract liabilities, net of current		520,544	529,701	504,900
Deferred income tax liability		109,356	94,835	102,571
Retained earnings		962,765	853,928	877,111
Accumulated other comprehensive loss		(106,613)	(93,700)	(93,691)
Contract liabilities		829,200	840,400
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract liabilities result of change in accounting for contracts containing material rights the client received due to having contract	$ 23,200
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net				256,595
Prepaid expenses and other current assets				47,220
Other assets				104,516
Contract liabilities, current				303,948
Contract liabilities, net of current				504,900
Deferred income tax liability				102,571
Retained earnings				877,111
Accumulated other comprehensive loss				$ (93,691)
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net		255,364
Prepaid expenses and other current assets		53,153
Other assets		117,013
Contract liabilities, current		299,617
Contract liabilities, net of current		523,175
Deferred income tax liability		110,101
Retained earnings		965,505
Accumulated other comprehensive loss		(106,642)
Adoption Adjustments | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net		(1,101)	(941)
Prepaid expenses and other current assets		362	(965)
Other assets		2,367	2,546
Contract liabilities, current		9,042	6,767
Contract liabilities, net of current		(2,631)	24,801
Deferred income tax liability		(745)	(7,736)
Retained earnings		(2,740)	(23,183)
Accumulated other comprehensive loss		$ 29	(9)
Contract liabilities			$ 31,600