Net Income Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 06, 2017	Mar. 04, 2016
Stock Repurchase Program, Authorized Amount	$ 1,000,000		$ 1,000,000		$ 500,000	$ 500,000
Treasury Stock Acquired, Average Cost Per Share			$ 56.37
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 430,428		$ 430,428
Common Class A [Member]
Treasury Stock, Shares, Acquired			6,143,745
Employee Stock Option [Member]
Antidilutive securities excluded from EPS	170,000		170,000
Performance Shares [Member]
Antidilutive securities excluded from EPS	960,000	137,000	960,000	137,000