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Net Income Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jun. 06, 2017	Mar. 07, 2016
Stock Repurchase Program, Authorized Amount	$ 1,000.0		$ 500.0	$ 500.0
Treasury Stock Acquired, Average Cost Per Share	$ 50.77
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 258.9
Common Class A [Member]
Treasury Stock, Shares, Acquired	1,600,000
Employee Stock Option [Member]
Antidilutive securities excluded from EPS	388,000
Restricted Stock Units (RSUs) [Member]
Antidilutive securities excluded from EPS	170,000
Performance Shares [Member]
Antidilutive securities excluded from EPS	195,000	175,000