Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income (loss)	$ 231.7	$ 460.0	$ 459.5	$ 402.5	$ 842.9	$ 813.2	$ 314.8	$ 353.6	$ 1,553.7	$ 2,324.5	$ 1,361.8
Subtract:
Net income (loss) attributable to noncontrolling interest									7.2	14.1	45.3
Total									$ 1,546.5	$ 2,310.4	$ 1,316.5
Weighted-average shares outstanding:
Basic									285.8	288.9	289.4
Dilutive effects:
Diluted									288.8	293.1	292.7
Net income (loss) per common share:
Basic	$ 0.84	$ 1.60	$ 1.59	$ 1.37	$ 2.91	$ 2.80	$ 1.07	$ 1.21	$ 5.41	$ 8.00	$ 4.55
Diluted	$ 0.83	$ 1.59	$ 1.58	$ 1.36	$ 2.87	$ 2.76	$ 1.06	$ 1.19	$ 5.36	$ 7.88	$ 4.50
Nonqualified Stock Options
Dilutive effects:
Stock-based compensation awards									1.0	2.0	1.3
Restricted Stock Units
Dilutive effects:
Stock-based compensation awards									1.7	2.0	1.7
Performance Share Awards
Dilutive effects:
Stock-based compensation awards									0.3	0.2	0.3