Stockholders' Deficit (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Feb. 08, 2018
Treasury Stock Repurchase Programs [Line Items]
Additional share repurchase amount authorized	$ 585.8
Remaining common stock available for repurchase	$ 938.3	$ 414.2
Common stock, held in treasury	229.0
Common stock repurchased	$ 8,900.0
Share Buyback Program [Member]
Treasury Stock Repurchase Programs [Line Items]
Common stock authorized to repurchase		$ 1,000.0
Common stock repurchase, share	1.1
Average stock price	$ 114.81
Aggregate cost of share	$ 124.9
Restricted Stock Units [Member]
Treasury Stock Repurchase Programs [Line Items]
Common stock repurchase, share	0.2
Average stock price	$ 117.89
Aggregate cost of share	$ 27.8