Basis of Condensed Consolidated Financial Statements (Impact of Adoption of New Accounting Standard on Balance Sheet) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net	$ 244,046	$ 255,654	$ 256,595
Prepaid expenses and other current assets	45,202	46,255	47,220
Income tax receivable	8,053		7,649
Other assets	110,721	107,062	104,516
Accounts payable and other accrued expenses	171,030		145,655
Contract liabilities, current	310,590	310,715	303,948
Contract liabilities, net of current	527,791	529,701	504,900
Deferred income tax liability	103,695	94,835	102,571
Retained earnings	881,798	853,928	877,111
Accumulated other comprehensive loss	(93,384)	$ (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	244,303
Prepaid expenses and other current assets	47,308
Income tax receivable	8,044
Other assets	108,275
Accounts payable and other accrued expenses	171,220
Contract liabilities, current	302,351
Contract liabilities, net of current	504,906
Deferred income tax liability	111,431
Retained earnings	904,912
Accumulated other comprehensive loss	(93,391)
Accounting Standards Update 2014-09 | Effect of Change Higher/(Lower)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net	(257)		(941)
Prepaid expenses and other current assets	(2,106)		(965)
Income tax receivable	9
Other assets	2,446		2,546
Accounts payable and other accrued expenses	(190)
Contract liabilities, current	8,239		6,767
Contract liabilities, net of current	22,885		24,801
Deferred income tax liability	(7,736)		(7,736)
Retained earnings	(23,114)		(23,183)
Accumulated other comprehensive loss	$ 7		$ (9)