Business Overview and Summary of Significant Accounting Policies (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
Business Overview and Summary of Significant Accounting Policies [Abstract]
Net income used for basic calculation	$ 889.0	$ 467.6	$ 459.8	$ 587.9	$ (45.0)	$ 330.9	$ (457.4)	$ 249.1	$ 2,404.3	$ 77.6	$ 443.3
Shares used in computing earnings per common share—basic									223.2	222.7	223.9
Weighted-average effect of dilutive securities:
Stock awards									1.6	1.8	1.5
Shares used in computing earnings per common share—diluted									224.8	224.5	225.4
Earnings per common share
Basic (in dollars per share)	$ 4.02	$ 2.09	$ 2.05	$ 2.63	$ (0.20)	$ 1.48	$ (2.05)	$ 1.12	$ 10.77	$ 0.35	$ 1.98
Diluted (in dollars per share)	$ 4.00	$ 2.08	$ 2.04	$ 2.61	$ (0.20)	$ 1.47	$ (2.05)	$ 1.11	$ 10.70	$ 0.35	$ 1.97
Antidilutive securities excluded from computation of EPS									3.0	2.8	4.0