Earnings Per Common Share - Computation of EPS (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net income (loss)	$ (18.3)	$ (27.3)	$ 114.2	$ 7.0	$ (28.1)	$ 9.3	$ 0.8	$ (36.7)	$ 75.6	$ (54.7)	$ 11.2
Preferred stock dividends									10.4	10.4	10.4
Net income (loss) applicable to common shareowners - basic and diluted									$ 65.2	$ (65.1)	$ 0.8
Denominator:
Weighted-average common shares outstanding - basic									208.5	205.9	197.0
Warrants									0	0	4.5
Stock-based compensation arrangements									1.1	0	3.2
Weighted-average common shares outstanding - diluted									209.6	205.9	204.7
Basic and diluted earnings (loss) per common share					$ (0.15)	$ 0.03	$ (0.01)	$ (0.19)	$ 0.31	$ (0.32)	$ 0.00