Basis of Condensed Consolidated Financial Statements (Impact of Adoption of New Accounting Standard on Balance Sheet) (Details) - USD ($) $ in Thousands	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
Income tax receivable	8,984		7,649
Other assets	119,380	107,428	104,882
Accounts payable and other accrued expenses	157,580		145,655
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)
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	255,364
Prepaid expenses and other current assets	53,153
Income tax receivable	7,985
Other assets	117,013
Accounts payable and other accrued expenses	157,908
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)
Accounting Standards Update 2014-09 | Effect of Change Higher/(Lower)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net	(1,101)	(941)
Prepaid expenses and other current assets	362	(965)
Income tax receivable	999
Other assets	2,367	2,546
Accounts payable and other accrued expenses	(328)
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)