Stockholders' Equity Statement of Changes in Common Shares Outstanding (Details) - shares shares in Millions	12 Months Ended			47 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Class A Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Purchases of treasury stock	(36.9)	(38.3)	(44.5)	(148.9)
Common Stock | Class A Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	1,095.0	1,115.4	1,148.8
Purchases of treasury stock	(36.9)	(38.3)	(44.5)
Share-based payments	2.3	2.0	2.9
Conversion of Class B to Class A common stock	2.0	15.9	8.2
Balance	1,062.4	1,095.0	1,115.4	1,062.4
Common Stock | Class B Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	21.3	37.2	45.4
Conversion of Class B to Class A common stock	(2.0)	(15.9)	(8.2)
Balance	19.3	21.3	37.2	19.3