Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator:
Net income	$ 2.1	$ 77.6	$ 62.5	$ 84.6
Preferred stock dividends	2.6	2.6	5.2	5.2
Net income applicable to common shareowners - basic and diluted	$ (0.5)	$ 75.0	$ 57.3	$ 79.4
Denominator:
Weighted average common shares outstanding - basic	42.2	42.0	42.1	42.0
Stock-based compensation arrangements	0.0	0.1	0.2	0.1
Weighted average common shares outstanding - diluted	42.2	42.1	42.3	42.1
Basic earnings per common share	$ (0.01)	$ 1.79	$ 1.36	$ 1.89
Diluted earnings per common share	$ (0.01)	$ 1.78	$ 1.35	$ 1.89
Stock Compensation Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0.3	0.4		0.5
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0.9	0.9	0.9	0.9