Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income (loss)	$ 315.3	$ 284.6	$ 392.1	$ 452.1	$ 231.7	$ 460.0	$ 459.5	$ 402.5	$ 1,444.1	$ 1,553.7	$ 2,324.5
Subtract:
Net income (loss) attributable to noncontrolling interest									49.9	7.2	14.1
Total									$ 1,394.2	$ 1,546.5	$ 2,310.4
Weighted-average shares outstanding:
Basic									278.6	285.8	288.9
Dilutive effects:
Diluted									281.0	288.8	293.1
Net income (loss) per common share:
Basic	$ 1.08	$ 0.99	$ 1.38	$ 1.54	$ 0.84	$ 1.60	$ 1.59	$ 1.37	$ 5.00	$ 5.41	$ 8.00
Diluted	$ 1.07	$ 0.98	$ 1.37	$ 1.53	$ 0.83	$ 1.59	$ 1.58	$ 1.36	$ 4.96	$ 5.36	$ 7.88
Nonqualified Stock Options
Dilutive effects:
Stock-based compensation awards									0.9	1.0	2.0
Restricted Stock Units
Dilutive effects:
Stock-based compensation awards									1.4	1.7	2.0
Performance Share Awards
Dilutive effects:
Stock-based compensation awards									0.1	0.3	0.2