Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ 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
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues	$ 11,472	$ 11,069	$ 11,144	$ 10,990	$ 9,481	$ 10,465	$ 10,099	$ 9,770	$ 44,675	$ 39,815	$ 39,407
Net income (loss) applicable to common shareholders	$ 1,707	$ 889	$ 821	$ 1,261	$ (585)	$ 942	$ 678	$ 977	$ 4,678	$ 2,012	$ 3,438
Earnings per common share - Basic (in dollars per share)	$ 5.32	$ 2.71	$ 2.47	$ 3.79	$ (1.71)	$ 2.72	$ 1.94	$ 2.76	$ 14.25	$ 5.78	$ 9.50
Earnings per common share - Diluted (in dollars per share)	$ 5.23	$ 2.67	$ 2.44	$ 3.74	$ (1.71)	$ 2.68	$ 1.91	$ 2.71	$ 14.03	$ 5.70	$ 9.35
Impact of change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Net income (loss) applicable to common shareholders	$ 240	$ (140)	$ (69)	$ 5	$ (273)	$ 109	$ 41	$ 31
Earnings per common share - Basic (in dollars per share)	$ 0.75	$ (0.43)	$ (0.21)	$ 0.01	$ (0.80)	$ 0.31	$ 0.12	$ 0.09
Earnings per common share - Diluted (in dollars per share)	$ 0.73	$ (0.42)	$ (0.20)	$ 0.02	$ (0.80)	$ 0.31	$ 0.11	$ 0.08