Income from Continuing Operations Per Unrestricted Share - Reconciliation of Numerator and Denominator in Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Income from Continuing Operations	$ 5.0	$ 92.3	$ 37.5	$ 53.6	$ 531.1	$ 188.4	$ 119.9
Dilutive Effect on Income of Equity-based Compensation Equivalent Shares					0.0	0.0	0.0
Diluted Income from Continuing Operations Attributed to Unrestricted Shares					$ 529.4	$ 187.4	$ 119.1
Weighted-average Unrestricted Shares Outstanding (in shares)					65,880,900	58,149,400	51,345,600
Equity-based Compensation Equivalent Shares (in shares)					667,200	602,500	232,300
Weighted-average Unrestricted Shares and Equivalent Shares Outstanding Assuming Dilution (in shares)					66,548,100	58,751,900	51,577,900
Basic Income from Continuing Operations Per Unrestricted Share (in dollars per share)	$ 0.08	$ 1.42	$ 0.73	$ 1.03	$ 8.04	$ 3.22	$ 2.32
Diluted Income from Continuing Operations Per Unrestricted Share (in dollars per share)	$ 0.08	$ 1.40	$ 0.73	$ 1.02	$ 7.96	$ 3.19	$ 2.31
Common Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Income from Continuing Operations					$ 529.4	$ 187.4	$ 119.1
Participating Awards
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Income from Continuing Operations					$ 1.7	$ 1.0	$ 0.8