Summary of Significant Accounting Policies Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income [Abstract]
Net income (loss)	$ 17.5	$ 143.1	$ 63.6	$ 26.0	$ 4.7	$ 146.4	$ 58.1	$ (9.3)	$ 250.2	$ 199.9	$ 174.2
Less: net income (loss) attributable to noncontrolling interest									0	0	(0.2)
Less: preferred stock dividend requirements									1.6	1.6	1.6
Earnings (loss) available for common shareholders									$ 248.6	$ 198.3	$ 172.8
Common Shares Outstanding [Abstract]
Average number of common shares outstanding (in shares)									153.5	145.5	135.6
Add: effect of dilutive securities (in shares)									0.2	1.7	3.1
Diluted average number of common shares outstanding (in shares)									153.7	147.2	138.7
Earnings Per Share [Abstract]
Earnings Per Share, Basic									$ 1.62	$ 1.36	$ 1.27
Earnings Per Share, Diluted									$ 1.62	$ 1.35	$ 1.25
Earnings Per Share, Basic and Diluted	$ 0.11	$ 0.93	$ 0.41	$ 0.17	$ 0.03	$ 0.95	$ 0.41	$ (0.07)