Earnings Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Common Share
Net income (loss)	$ 191.3	$ 74.5	$ 594.3	$ 532.5
Subtract:
Net income (loss) attributable to noncontrolling interest	3.4	(5.6)	15.3	36.6
Preferred stock dividends	8.2	8.2	24.7	24.7
Net income (loss) available to common stockholders	$ 179.7	$ 71.9	$ 554.3	$ 471.2
Weighted-average shares outstanding
Basic	294.7	311.9	298.6	317.7
Dilutive effects:
Stock options	1.0	1.0	1.0	1.3
Restricted stock units	1.5	1.5	1.4	1.5
Performance share awards	0.3	0.4	0.4	0.4
Diluted	297.5	314.8	301.4	320.9
Net income (loss) per common share:
Basic	$ 0.61	$ 0.23	$ 1.86	$ 1.48
Diluted	$ 0.60	$ 0.23	$ 1.84	$ 1.47