Earnings Per Share Reconciliation (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share [Abstract]
Net income available to common stockholders for purposes of computing basic earnings per share									$ 377.1	$ 328.7	$ 169.2
Effect of dividends on conversion of convertible preferred stock									0	1.3	10.8
Net income available to common stockholders for purposes of computing diluted earnings per share									$ 377.1	$ 330.0	$ 180.0
Weighted-average number of shares outstanding (in thousands):
Basic shares									109,712	108,208	100,054
Additional weighted average shares attributable to:
Convertible preferred stock									0	1,245	7,000
Stock options and nonvested shares									368	377	480
Diluted									110,080	109,830	107,534
Basic earnings per share	$ 0.84	$ 0.82	$ 1.10	$ 0.68	$ 0.87	$ 0.91	$ 0.65	$ 0.60	$ 3.44	$ 3.04	$ 1.69
Diluted earnings per share	$ 0.83	$ 0.82	$ 1.09	$ 0.68	$ 0.87	$ 0.91	$ 0.64	$ 0.58	$ 3.43	$ 3.00	$ 1.67