Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Retained earnings	$ 3,786,521			$ 3,509,484
Contract assets	7,307
Deferred income tax expense	8,160	$ 67,497
Effect of Change Higher/(Lower) [Member] | Accounting Standards Update 2014-09 [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Retained earnings	123,748		$ 130,700
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Change in Estimate of Transaction Price	99,900
Contract assets	7,307		$ 7,300
Sales Commissions and Fees	66,400
Deferred income tax expense	$ 42,900