Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions																			2 Months Ended	3 Months Ended			12 Months Ended
	May 15, 2020	Apr. 30, 2020	Mar. 13, 2020	Mar. 01, 2020	Dec. 10, 2019	Dec. 01, 2019	Nov. 15, 2019	Oct. 30, 2019	Oct. 09, 2019	Sep. 09, 2019	Sep. 01, 2019	Aug. 15, 2019	Jul. 10, 2019	Jun. 30, 2019	Apr. 12, 2019	Mar. 01, 2019	Dec. 01, 2018	Apr. 29, 2018	Dec. 31, 2019	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares of preferred stock authorized to issue, in one or more series (in shares)																			200,000,000	200,000,000			200,000,000	200,000,000
Preferred stock, par value (in dollars per share)																			$ 1	$ 1			$ 1	$ 1
Shares (in shares)																			2,699,250	3,006,250			3,006,250	2,699,250
Carrying value																			$ 26,993	$ 30,063			$ 30,063	$ 26,993
Liquidation value and redemption price per share (in dollars per share)																				$ 10,000			$ 10,000
Aggregate liquidation value																				$ 30,500			$ 30,500
Series P
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			0	0			0	0
Carrying value																			$ 0	$ 0			$ 0	$ 0
Issue date																							Feb. 05, 2013
Contractual rate in effect						5.45%																	0.00%
Dividend declared per share (in dollars per share)																							$ 0	$ 545.00	$ 545.00
Preferred stock, shares redeemed						$ 900
Series P | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Mar. 01, 2018
Series T
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			0	0			0	0
Carrying value																			$ 0	$ 0			$ 0	$ 0
Issue date																							Jan. 30, 2014
Contractual rate in effect																6.70%							0.00%
Dividend declared per share (in dollars per share)																							$ 0	$ 167.50	670.00
Preferred stock, shares redeemed																$ 925
Series T | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Mar. 01, 2019
Series W
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			0	0			0	0
Carrying value																			$ 0	$ 0			$ 0	$ 0
Issue date																							Jun. 23, 2014
Contractual rate in effect											6.30%												0.00%
Dividend declared per share (in dollars per share)																							$ 0	$ 472.50	630.00
Preferred stock, shares redeemed											$ 880
Series W | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Sep. 01, 2019
Series Y
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			143,000	0			0	143,000
Carrying value																			$ 1,430	$ 0			$ 0	$ 1,430
Issue date																							Feb. 12, 2015
Contractual rate in effect				6.125%																			0.00%
Dividend declared per share (in dollars per share)																							$ 153.13	$ 612.52	612.52
Preferred stock, shares redeemed				$ 1,430
Series Y | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Mar. 01, 2020
Series AA
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			142,500	142,500			142,500	142,500
Carrying value																			$ 1,425	$ 1,425			$ 1,425	$ 1,425
Issue date																							Jun. 04, 2015
Contractual rate in effect																							6.10%
Dividend declared per share (in dollars per share)																							$ 610.00	$ 610.00	610.00
Series AA | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Sep. 01, 2020
Series BB
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			115,000	115,000			115,000	115,000
Carrying value																			$ 1,150	$ 1,150			$ 1,150	$ 1,150
Issue date																							Jul. 29, 2015
Contractual rate in effect																							6.15%
Dividend declared per share (in dollars per share)																							$ 615.00	$ 615.00	615.00
Series BB | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Sep. 01, 2020
Series DD
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			169,625	169,625			169,625	169,625
Carrying value																			$ 1,696	$ 1,696			$ 1,696	$ 1,696
Issue date																							Sep. 21, 2018
Contractual rate in effect																							5.75%
Dividend declared per share (in dollars per share)																	$ 111.81						$ 575.00	$ 575.00	111.81
Series DD | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Dec. 01, 2023
Series EE
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			185,000	185,000			185,000	185,000
Carrying value																			$ 1,850	$ 1,850			$ 1,850	$ 1,850
Issue date																							Jan. 24, 2019
Contractual rate in effect																							6.00%
Dividend declared per share (in dollars per share)									$ 150.00				$ 150.00		$ 211.67								$ 600.00	$ 511.67
Series EE | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Mar. 01, 2024
Series GG
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			90,000	90,000			90,000	90,000
Carrying value																			$ 900	$ 900			$ 900	$ 900
Issue date																							Nov. 07, 2019
Contractual rate in effect																							4.75%
Dividend declared per share (in dollars per share)																			$ 0				$ 506.67
Series GG | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Dec. 01, 2024
Series I
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			293,375	293,375			293,375	293,375
Carrying value																			$ 2,934	$ 2,934			$ 2,934	$ 2,934
Issue date																							Apr. 23, 2008
Contractual rate in effect																		7.90%
Dividend declared per share (in dollars per share)																		$ 395.00					$ 428.03	$ 593.23	646.38
Preferred stock, shares redeemed								$ 1,370
Series I | Three-month LIBOR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							three-month LIBOR
Preferred stock dividend rate, variable, basis spread																							0.00035%
Series I | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Apr. 30, 2018
Series Q
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			150,000	150,000			150,000	150,000
Carrying value																			$ 1,500	$ 1,500			$ 1,500	$ 1,500
Issue date																							Apr. 23, 2013
Contractual rate in effect																							5.15%
Dividend declared per share (in dollars per share)																							$ 515.00	$ 515.00	515.00
Series Q | Three-month LIBOR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							three-month LIBOR
Preferred stock dividend rate, variable, basis spread																							0.0325%
Series Q | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							May 01, 2023
Series R
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			150,000	150,000			150,000	150,000
Carrying value																			$ 1,500	$ 1,500			$ 1,500	$ 1,500
Issue date																							Jul. 29, 2013
Contractual rate in effect																							6.00%
Dividend declared per share (in dollars per share)																							$ 600.00	$ 600.00	600.00
Series R | Three-month LIBOR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							three-month LIBOR
Preferred stock dividend rate, variable, basis spread																							0.00033%
Series R | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Aug. 01, 2023
Series S
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			200,000	200,000			200,000	200,000
Carrying value																			$ 2,000	$ 2,000			$ 2,000	$ 2,000
Issue date																							Jan. 22, 2014
Contractual rate in effect																							6.75%
Dividend declared per share (in dollars per share)																							$ 675.00	$ 675.00	675.00
Series S | Three-month LIBOR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							three-month LIBOR
Preferred stock dividend rate, variable, basis spread																							0.0378%
Series S | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Feb. 01, 2024
Series U
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			100,000	100,000			100,000	100,000
Carrying value																			$ 1,000	$ 1,000			$ 1,000	$ 1,000
Issue date																							Mar. 10, 2014
Contractual rate in effect																							6.125%
Dividend declared per share (in dollars per share)																							$ 612.50	$ 612.50	612.50
Series U | Three-month LIBOR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							three-month LIBOR
Preferred stock dividend rate, variable, basis spread																							0.0333%
Series U | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Apr. 30, 2024
Series V
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			250,000	250,000			250,000	250,000
Carrying value																			$ 2,500	$ 2,500			$ 2,500	$ 2,500
Issue date																							Jun. 09, 2014
Contractual rate in effect														5.00%
Dividend declared per share (in dollars per share)							$ 139.98					$ 144.11		$ 250.00									$ 436.85	$ 534.09	500.00
Series V | Three-month LIBOR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							three-month LIBOR
Preferred stock dividend rate, variable, basis spread																							0.0332%
Series V | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Jul. 01, 2019
Series X
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			160,000	160,000			160,000	160,000
Carrying value																			$ 1,600	$ 1,600			$ 1,600	$ 1,600
Issue date																							Sep. 23, 2014
Contractual rate in effect																							6.10%
Dividend declared per share (in dollars per share)																							$ 610.00	$ 610.00	610.00
Series X | Three-month LIBOR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							three-month LIBOR
Preferred stock dividend rate, variable, basis spread																							0.0333%
Series X | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Oct. 01, 2024
Series Z
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			200,000	200,000			200,000	200,000
Carrying value																			$ 2,000	$ 2,000			$ 2,000	$ 2,000
Issue date																							Apr. 21, 2015
Contractual rate in effect		5.30%
Dividend declared per share (in dollars per share)																							$ 453.52	$ 530.00	530.00
Series Z | Three-month LIBOR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							three-month LIBOR
Preferred stock dividend rate, variable, basis spread																							0.00038%
Series Z | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							May 01, 2020
Series CC
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			125,750	125,750			125,750	125,750
Carrying value																			$ 1,258	$ 1,258			$ 1,258	$ 1,258
Issue date																							Oct. 20, 2017
Contractual rate in effect																							4.625%
Dividend declared per share (in dollars per share)																							$ 462.50	$ 462.50	$ 462.50
Series CC | Three-month LIBOR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							three-month LIBOR
Preferred stock dividend rate, variable, basis spread																							0.0258%
Series CC | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Nov. 01, 2022
Series FF
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			225,000	225,000			225,000	225,000
Carrying value																			$ 2,250	$ 2,250			$ 2,250	$ 2,250
Issue date																							Jul. 31, 2019
Contractual rate in effect																							5.00%
Dividend declared per share (in dollars per share)					$ 125.00					$ 126.39													$ 500.00	$ 251.39
Series FF | Term SOFR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							Term SOFR
Preferred stock dividend rate, variable, basis spread																							0.0338%
Series FF | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Aug. 01, 2024
Series HH
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			0	300,000			300,000	0
Carrying value																			$ 0	$ 3,000			$ 3,000	$ 0
Issue date																							Jan. 23, 2020
Contractual rate in effect																							4.60%
Dividend declared per share (in dollars per share)			$ 125.22																	$ 115.00	$ 115.00	$ 115.00	$ 470.22
Series HH | Term SOFR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							Term SOFR
Preferred stock dividend rate, variable, basis spread																							0.03125%
Series HH | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Feb. 01, 2025
Series II
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)																			0	150,000			150,000	0
Carrying value																			$ 0	$ 1,500			$ 1,500	$ 0
Issue date																							Feb. 24, 2020
Contractual rate in effect																							4.00%
Dividend declared per share (in dollars per share)	$ 141.11																			$ 100.00	$ 100.00		$ 341.11
Series II | Term SOFR
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate, variable, description of basis																							Term SOFR
Preferred stock dividend rate, variable, basis spread																							0.02745%
Series II | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date																							Apr. 01, 2025