Stockholders' Equity - Stock Repurchases - Tabular Disclosure (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended				63 Months Ended
	Dec. 31, 2015	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2020
Stockholders' Equity
Share repurchases made in open market (in shares)	345,044	60,901	844,872	280,140	1,828,183	3,359,140
Average Price Paid per Share (in dollars per share)	$ 53.46	$ 61.15	$ 74.59	$ 77.67	$ 58.40
Aggregate Cost	$ 18.4	$ 3.7	$ 63.0	$ 21.8	$ 106.8	$ 213.7