Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Retained earnings	$ 4,052,211	$ 4,052,211			$ 3,509,484
Deferred income tax expense		2,508	$ 32,367
Effect of Change Higher/(Lower) [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Retained earnings	137,383	$ 137,383		$ 130,700
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Change in Estimate of Transaction Price	99,900
Contract assets				$ 7,300
Sales Commissions and Fees	66,400
Deferred income tax expense	$ 42,900