Stockholders' Equity Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Stock Issued During Period, Value, New Issues		$ 2,525.6
Capital Stock Activity [Roll Forward]
Stock Issued During Period, Shares, New Issues		29.9
Common stock issued, Class A
Capital Stock Activity [Roll Forward]
Common stock issued, beginning balance (in shares)	2.6	2.6	2.6
Stock Issued During Period, Shares, New Issues		0.0
Shares issued under equity compensation plans	0.0	0.0	0.0
Shares exchanged for common stock	0.0	0.0	0.0
Common stock issued, ending balance (in shares)	2.6	2.6	2.6
Common stock, par value	$ 0.01	$ 0.01
Common stock issued, Class B
Class of Stock [Line Items]
Stock Issued During Period, Value, New Issues		$ 0.3
Capital Stock Activity [Roll Forward]
Common stock issued, beginning balance (in shares)	203.7	172.5	169.9
Stock Issued During Period, Shares, New Issues		29.9
Shares issued under equity compensation plans	0.5	0.5	1.0
Shares exchanged for common stock	0.5	0.8	1.6
Common stock issued, ending balance (in shares)	204.7	203.7	172.5
Common stock, par value	$ 0.01	$ 0.01
Exchangeable shares issued, Class A
Capital Stock Activity [Roll Forward]
Exchangeable stock issued, beginning balance (in shares)	2.9	2.9	2.9
Stock Issued During Period, Shares, New Issues		0.0
Shares issued under equity compensation plans	0.0	0.0	0.0
Shares exchanged for common stock	0.0	0.0	0.0
Exchangeable stock issued, ending balance (in shares)	2.9	2.9	2.9
Exchangeable shares issued, Class B
Capital Stock Activity [Roll Forward]
Exchangeable stock issued, beginning balance (in shares)	15.2	16.0	17.6
Stock Issued During Period, Shares, New Issues		0.0
Shares issued under equity compensation plans	0.0	0.0	0.0
Shares exchanged for common stock	(0.5)	(0.8)	(1.6)
Exchangeable stock issued, ending balance (in shares)	14.7	15.2	16.0