Basis of Presentation - Share Repurchase Program (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions						12 Months Ended		24 Months Ended
	May 11, 2018	Mar. 15, 2018	Feb. 15, 2018	Nov. 17, 2017	May 12, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)	0.0	1.0	2.0	2.0	0.0
Aggregate purchase price						$ 141.5	$ 136.7	$ 278.2
Average cost per share (usd per share)						$ 47.15	$ 42.87	$ 44.94
Remaining authorized shares for repurchase (shares)						3.8	6.8	3.8
Common stock
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)						3.0	2.0	5.0
Common Class A
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)						0.0		1.2