Earnings Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Common Share
Net income (loss)	$ 171.9	$ 66.3	$ 289.9	$ 223.1	$ 218.1	$ 151.3	$ 144.2	$ 203.6	$ 751.2	$ 717.2	$ 645.7
Subtract:
Net income (loss) attributable to noncontrolling interest									36.2	17.9	23.0
Preferred stock dividends									33.0	33.0	33.0
Net income (loss) available to common stockholders	$ 164.0	$ 63.7	$ 258.0	$ 196.3	$ 199.3	$ 142.2	$ 134.0	$ 190.8	$ 682.0	$ 666.3	$ 589.7
Weighted-average shares outstanding
Basic									314.5	320.3	297.3
Dilutive effects:
Stock options									1.2	0.9	0.4
Performance share awards									0.4	0.3
Restricted stock units									1.5	1.5	1.2
Diluted									317.6	323.0	298.9
Net income (loss) per common share:
Basic	$ 0.54	$ 0.20	$ 0.81	$ 0.61	$ 0.62	$ 0.44	$ 0.42	$ 0.60	$ 2.17	$ 2.08	$ 1.98
Diluted	$ 0.54	$ 0.20	$ 0.80	$ 0.60	$ 0.62	$ 0.44	$ 0.42	$ 0.59	$ 2.15	$ 2.06	$ 1.97