Basis of Condensed Consolidated Financial Statements (Impact of Adoption of New Accounting Standard on Balance Sheet) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net	$ 256,225	$ 255,654	$ 256,595
Prepaid expenses and other current assets	52,438	46,255	47,220
Income tax receivable	16,332		7,649
Deferred income tax assets	127		366
Other assets	114,197	107,062	104,516
Accounts payable and other accrued expenses	159,975		145,655
Contract liabilities, current	322,700	310,715	303,948
Contract liabilities, net of current	511,837	529,701	504,900
Deferred income tax liability	106,815	94,835	102,571
Retained earnings	940,314	853,928	877,111
Accumulated other comprehensive loss	(102,892)	(93,700)	(93,691)
Balances Without Adoption Adjustments
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)
Accounting Standards Update 2014-09 | Balances Without Adoption Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net	256,501
Prepaid expenses and other current assets	53,632
Income tax receivable	15,345
Deferred income tax assets	133
Other assets	111,675
Accounts payable and other accrued expenses	160,167
Contract liabilities, current	314,808
Contract liabilities, net of current	512,820
Deferred income tax liability	107,872
Retained earnings	943,962
Accumulated other comprehensive loss	(102,913)
Accounting Standards Update 2014-09 | Effect of Change Higher/(Lower)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net	(276)	(941)
Prepaid expenses and other current assets	(1,194)	(965)
Income tax receivable	987
Deferred income tax assets	(6)
Other assets	2,522	2,546
Accounts payable and other accrued expenses	(192)
Contract liabilities, current	7,892	6,767
Contract liabilities, net of current	(983)	24,801
Deferred income tax liability	(1,057)	(7,736)
Retained earnings	(3,648)	(23,183)
Accumulated other comprehensive loss	$ 21	$ (9)