Stock Incentive Plan (Schedule of Share-based Compensation, Unearned Compensation Related to RSU's) (Details) - Restricted Stock Units (RSUs) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Changes In Unearned Compensation [Roll Forward]
Unearned compensation at beginning of year	$ 115,965	$ 103,538	$ 88,015
RSUs granted, net of cancellations	52,897	52,697	50,442
RSUs expensed	38,796	(35,585)	(30,691)
RSUs forfeitures	7,156	(4,685)	(4,228)
Unearned compensation at end of year	$ 122,910	$ 115,965	$ 103,538