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Net Income Per Share (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jun. 13, 2018	Jun. 06, 2017	Mar. 07, 2016
Stock Repurchase Program, Authorized Amount	$ 1,500.0		$ 500.0	$ 500.0	$ 500.0
Common Class A [Member]
Treasury Stock, Shares, Acquired	5.1
Treasury Stock Acquired, Average Cost Per Share	$ 52.53
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 575.2
Restricted Stock Units (RSUs) [Member]
Antidilutive securities excluded from EPS	1.5	1.0
Employee Stock Option [Member]
Antidilutive securities excluded from EPS	0.4