Shareholders' Equity - Share Repurchase and Dividends (Details) - USD ($) $ / shares in Units, $ in Millions										7 Months Ended	8 Months Ended	12 Months Ended
	Dec. 22, 2017	Sep. 22, 2017	Jun. 23, 2017	Mar. 24, 2017	Dec. 23, 2016	Sep. 23, 2016	Jun. 24, 2016	Mar. 24, 2016	Feb. 22, 2016	Nov. 30, 2015	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Shareholders' Equity
Repurchases to date (in shares)											16,009,099	16,009,099	10,698,195
Repurchases to date											$ 401.2	$ 401.2	$ 249.3
Dividend declared per common share (in dollars per share)									$ 0.04			$ 0.16	$ 0.16
Target dividend per annum (in dollars per share)												$ 0.16
Dividend paid	$ 6.3	$ 6.3	$ 6.4	$ 6.4	$ 6.4	$ 6.4	$ 6.5	$ 6.5				$ 25.4	$ 25.8
Anti-Dilutive Repurchase Program
Shareholders' Equity
Common stock repurchased during the period (in shares)										1,245,000
Aggregate cost of common stock repurchased during the period										$ 24.9
November 2015 Repurchase Program
Shareholders' Equity
Amount approved for repurchase of common stock													$ 225.0
Repurchases to date (in shares)													9,312,522
Repurchases to date													$ 225.0
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
Aggregate cost of common stock repurchased during the period											$ 152.2