Earnings Per Common Share - Computation of basic and diluted earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended														12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018	Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share Reconciliation [Abstract]
Net income		$ 21.1	$ 32.7		$ 31.8		$ 27.7		$ 27.7	$ 3.2		$ 22.6		$ 22.2		$ 113.3	$ 75.7	$ 89.7
Unvested restricted share participation																(1.1)	(0.2)	0.0
Net income per common share – basic																$ 112.2	$ 75.5	$ 89.7
Net income - basic (shares)	[1]															47.9	48.8	48.8
Net income - basic (EPS)		$ 0.44	$ 0.68	[1]	$ 0.66	[1]	$ 0.57	[1]	$ 0.56	$ 0.07	[1]	$ 0.46	[1]	$ 0.45	[1]	$ 2.34	$ 1.55	$ 1.84
Effect of dilutive securities:
Nonparticipating unvested restricted shares																$ 0.0	$ 0.0	$ 0.0
Nonparticipating unvested restricted shares (shares)																0.5	0.1	0.0
Net income per common share – diluted																$ 112.2	$ 75.5	$ 89.7
Net income - diluted (shares)	[1]															48.4	48.9	48.8
Net income - diluted (EPS)		$ 0.43	$ 0.67	[1]	$ 0.65	[1]	$ 0.56	[1]	$ 0.56	$ 0.07	[1]	$ 0.46	[1]	$ 0.45	[1]	$ 2.32	$ 1.54	$ 1.84

[1] For periods prior to the Separation, the denominator for basic and diluted net income per common share was calculated using the 48.8 million shares of common stock outstanding immediately following the Separation.