Total Equity and Other Comprehensive Income (Rollforward of Total Equity) (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2020
Common Stock [Abstract]
Equity, Beginning Balance	$ 54,041.0	$ 49,343.0
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member
Net income	1,826.0	915.0
Other comprehensive income (loss), net of tax	(1,480.0)	1,719.0
Dividends, Cash [Abstract]
Common	(493.0)	(503.0)
Preferred	(57.0)	(63.0)
Treasury stock activity	128.0	(1,310.0)
Other	(86.0)	(140.0)
Equity, Ending Balance	53,879.0	49,290.0	$ 49,343.0
Par value less than $.5 million at each date	$ 0.5	0.5	0.5	$ 0.5
Cumulative Effect, Period of Adoption, Adjustment
Common Stock [Abstract]
Equity, Beginning Balance		(671.0)
Dividends, Cash [Abstract]
Equity, Ending Balance			(671.0)
Cumulative Effect, Period of Adoption, Adjusted Balance
Common Stock [Abstract]
Equity, Beginning Balance		$ 48,672.0
Dividends, Cash [Abstract]
Equity, Ending Balance			$ 48,672.0
Common Stock
Common Stock [Abstract]
Beginning Balance (in shares)	424	433
Treasury stock activity, shares	1	(9)
Ending Balance, (in shares)	425	424	433
Equity, Beginning Balance	$ 2,713.0	$ 2,712.0
Dividends, Cash [Abstract]
Equity, Ending Balance	2,713.0	$ 2,712.0	$ 2,712.0
Common Stock | Cumulative Effect, Period of Adoption, Adjusted Balance
Common Stock [Abstract]
Beginning Balance (in shares)		433
Ending Balance, (in shares)			433
Equity, Beginning Balance		$ 2,712.0
Dividends, Cash [Abstract]
Equity, Ending Balance			$ 2,712.0
Capital Surplus - Preferred Stock
Common Stock [Abstract]
Equity, Beginning Balance	3,517.0	3,993.0
Dividends, Cash [Abstract]
Preferred stock discount accretion	1.0	1.0
Equity, Ending Balance	3,518.0	3,994.0	3,993.0
Capital Surplus - Preferred Stock | Cumulative Effect, Period of Adoption, Adjusted Balance
Common Stock [Abstract]
Equity, Beginning Balance		3,993.0
Dividends, Cash [Abstract]
Equity, Ending Balance			3,993.0
Capital Surplus - Common Stock and Other
Common Stock [Abstract]
Equity, Beginning Balance	12,367.0	12,376.0
Dividends, Cash [Abstract]
Treasury stock activity	69.0	49.0
Other	(75.0)	(131.0)
Equity, Ending Balance	12,361.0	12,294.0	12,376.0
Capital Surplus - Common Stock and Other | Cumulative Effect, Period of Adoption, Adjusted Balance
Common Stock [Abstract]
Equity, Beginning Balance		12,376.0
Dividends, Cash [Abstract]
Equity, Ending Balance			12,376.0
Retained Earnings
Common Stock [Abstract]
Equity, Beginning Balance	46,848.0	42,215.0
Net income	1,816.0	908.0
Dividends, Cash [Abstract]
Common	(493.0)	(503.0)
Preferred	(57.0)	(63.0)
Preferred stock discount accretion	(1.0)	(1.0)
Equity, Ending Balance	48,113.0	41,885.0	42,215.0
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
Common Stock [Abstract]
Equity, Beginning Balance		(671.0)
Dividends, Cash [Abstract]
Equity, Ending Balance			(671.0)
Retained Earnings | Cumulative Effect, Period of Adoption, Adjusted Balance
Common Stock [Abstract]
Equity, Beginning Balance		41,544.0
Dividends, Cash [Abstract]
Equity, Ending Balance			41,544.0
Accumulated Other Comprehensive Income (Loss)
Common Stock [Abstract]
Equity, Beginning Balance	2,770.0	799.0
Other comprehensive income (loss), net of tax	(1,480.0)	1,719.0
Dividends, Cash [Abstract]
Equity, Ending Balance	1,290.0	2,518.0	799.0
Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjusted Balance
Common Stock [Abstract]
Equity, Beginning Balance		799.0
Dividends, Cash [Abstract]
Equity, Ending Balance			799.0
Treasury Stock
Common Stock [Abstract]
Equity, Beginning Balance	(14,205.0)	(12,781.0)
Dividends, Cash [Abstract]
Treasury stock activity	59.0	(1,359.0)
Equity, Ending Balance	(14,146.0)	(14,140.0)	(12,781.0)
Treasury Stock | Cumulative Effect, Period of Adoption, Adjusted Balance
Common Stock [Abstract]
Equity, Beginning Balance		(12,781.0)
Dividends, Cash [Abstract]
Equity, Ending Balance			(12,781.0)
Non- controlling Interests
Common Stock [Abstract]
Equity, Beginning Balance	31.0	29.0
Net income	10.0	7.0
Dividends, Cash [Abstract]
Other	(11.0)	(9.0)
Equity, Ending Balance	$ 30.0	27.0	29.0
Non- controlling Interests | Cumulative Effect, Period of Adoption, Adjusted Balance
Common Stock [Abstract]
Equity, Beginning Balance		$ 29.0
Dividends, Cash [Abstract]
Equity, Ending Balance			$ 29.0