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								9 Months Ended	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Income from Continuing Operations	$ 97.5	$ 122.3	$ 126.1	$ 64.0					$ 188.4	$ 409.9	$ 531.1	$ 188.4
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									$ 187.4	$ 409.5	$ 529.4
Weighted-average Unrestricted Shares Outstanding (in shares)									58,149,400	65,636,100	65,880,900
Equity-based Compensation Equivalent Shares (in shares)									602,500	1,093,700	667,200
Weighted-average Unrestricted Shares and Equivalent Shares Outstanding Assuming Dilution (in shares)									58,751,900	66,729,800	66,548,100
Basic Income from Continuing Operations Per Unrestricted Share (in dollars per share)					$ 1.87	$ 1.93	$ 1.87	$ 2.38	$ 3.22	$ 6.24	$ 8.04	$ 3.22
Diluted Income from Continuing Operations Per Unrestricted Share (in dollars per share)					$ 1.85	$ 1.91	$ 1.84	$ 2.35	$ 3.19	$ 6.14	$ 7.96	$ 3.19
Common Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Income from Continuing Operations									$ 187.4	$ 409.5	$ 529.4
Participating Awards
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Income from Continuing Operations									$ 1.0	$ 0.4	$ 1.7