Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income (loss)	$ 842.9	$ 813.2	$ 314.8	$ 353.6	$ 338.2	$ 327.4	$ 327.0	$ 369.2	$ 2,324.5	$ 1,361.8	$ 1,253.2
Subtract:
Net income (loss) attributable to noncontrolling interest									14.1	45.3	19.2
Preferred stock dividends											16.5
Excess of redemption value over carrying value of preferred shares redeemed											8.2
Total									$ 2,310.4	$ 1,316.5	$ 1,209.3
Weighted-average shares outstanding:
Basic									288.9	289.4	294.4
Dilutive effects:
Diluted									293.1	292.7	298.0
Net income (loss) per common share:
Basic	$ 2.91	$ 2.80	$ 1.07	$ 1.21	$ 1.10	$ 1.07	$ 1.11	$ 1.26	$ 8.00	$ 4.55	$ 4.11
Diluted	$ 2.87	$ 2.76	$ 1.06	$ 1.19	$ 1.09	$ 1.06	$ 1.10	$ 1.25	$ 7.88	$ 4.50	$ 4.06
Nonqualified Stock Options
Dilutive effects:
Stock-based compensation awards									2.0	1.3	1.5
Restricted Stock Units
Dilutive effects:
Stock-based compensation awards									2.0	1.7	1.7
Performance Share Awards
Dilutive effects:
Stock-based compensation awards									0.2	0.3	0.4