Shareholders' Equity - Share Repurchase and Dividends (Details) - USD ($) $ / shares in Units, $ in Millions										8 Months Ended	12 Months Ended
	Dec. 21, 2018	Sep. 21, 2018	Jun. 22, 2018	Mar. 23, 2018	Dec. 22, 2017	Sep. 22, 2017	Jun. 23, 2017	Mar. 24, 2017	Feb. 22, 2016	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shareholders' Equity
Aggregate cost of common stock repurchased during the period											$ 0.2
Dividend declared per common share (in dollars per share)									$ 0.04		$ 0.16	$ 0.16	$ 0.16
Target dividend per annum (in dollars per share)											$ 0.16
Dividends paid, Aggregate Cost	$ 6.3	$ 6.3	$ 6.2	$ 6.3	$ 6.3	$ 6.3	$ 6.4	$ 6.4
May 2017 Repurchase Program
Shareholders' Equity
Amount approved for repurchase of common stock										$ 225.0		$ 225.0
Common stock repurchased during the period (in shares)										5,318,063	0
Aggregate cost of common stock repurchased during the period										$ 152.2