Earnings Per Common 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
Earnings Per Common Share
Net income (loss)	$ 250.4	$ 259.1	$ 236.6	$ 190.0	$ 226.7	$ 198.8	$ 178.6	$ 221.3	$ 936.1	$ 825.4	$ 674.5
Subtract:
Net income (loss) attributable to noncontrolling interest					3.5	3.4	2.7	9.2	23.4	18.8	36.2
Preferred stock dividends					8.3	8.2	8.3	8.2	33.0	33.0	33.0
Net income (loss) available to common stockholders	$ 233.4	$ 245.7	$ 222.3	$ 178.3	$ 214.9	$ 187.2	$ 167.6	$ 203.9	$ 879.7	$ 773.6	$ 605.3
Weighted-average shares outstanding
Basic									294.6	297.5	314.5
Dilutive effects:
Stock options									1.6	1.0	1.2
Restricted stock units									1.7	1.5	1.5
Performance share awards									0.3	0.4	0.4
Diluted									298.2	300.4	317.6
Net income (loss) per common share:
Basic	$ 0.79	$ 0.83	$ 0.76	$ 0.61	$ 0.72	$ 0.64	$ 0.56	$ 0.68	$ 2.99	$ 2.60	$ 1.92
Diluted	$ 0.78	$ 0.82	$ 0.75	$ 0.61	$ 0.72	$ 0.63	$ 0.56	$ 0.68	$ 2.95	$ 2.58	$ 1.91