Net Income Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 16, 2017	Mar. 04, 2016
Stock Repurchase Program, Authorized Amount	$ 1,000,000		$ 500,000	$ 500,000
Treasury Stock Acquired, Average Cost Per Share	$ 55.97
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 532,389
Common Class A [Member]
Treasury Stock, Shares, Acquired	4,366,070
Employee Stock Option [Member]
Antidilutive securities excluded from EPS	388,000
Performance Shares [Member]
Antidilutive securities excluded from EPS	962,000	505,000