Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings per share:
Net income	$ (9.3)	$ 15.0	$ 12.2	$ 8.6	$ 10.6	$ 9.6	$ 8.0	$ (4.9)	$ 26.5	$ 23.3	$ 18.5
Preferred stock dividends	(1.0)	(1.0)	(1.0)	(1.0)	(1.0)	(1.0)	(1.0)	(1.0)	(4.0)	(4.0)	(4.0)
Net income less preferred stock dividends	(10.3)	14.0	11.2	7.6	9.6	8.6	7.0	(5.9)	22.5	19.3	14.5
Income allocated to participating securities		(0.7)	(0.5)	(0.3)	(0.4)	(0.4)	(0.3)		(1.0)	(0.8)	(0.6)
Net income available to common stockholders	$ (10.3)	$ 13.3	$ 10.7	$ 7.3	$ 9.2	$ 8.2	$ 6.7	$ (5.9)	$ 21.5	$ 18.5	$ 13.9
Basic weighted average number of common shares outstanding									63.1	71.0	74.9
Weighted-average common share equivalents from stock options restricted stock and convertible preferred stock									0.7	0.5	1.0
Diluted weighted average number of common shares outstanding									63.8	71.5	75.9
Earnings (loss) per share:
Basic (in dollars per share)	$ (0.17)	$ 0.21	$ 0.17	$ 0.11	$ 0.13	$ 0.12	$ 0.09	$ (0.08)	$ 0.34	$ 0.26	$ 0.19
Diluted (in dollars per share)	$ (0.17)	$ 0.21	$ 0.17	$ 0.11	$ 0.13	$ 0.12	$ 0.09	$ (0.08)	$ 0.34	$ 0.26	$ 0.18
Number of options and non-vested restricted stock awards assumed to be repurchased under the treasury stock method									1.7	2.9	2.9
Convertible Preferred Stock
Earnings (loss) per share:
Securities excluded in the computation of diluted income per share (in shares)									3.1	3.1	3.1