Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Sep. 01, 2013	Dec. 31, 2015	Dec. 31, 2014
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
Preferred stock, par value (in dollars per share)		$ 1.00	$ 1.00
Shares (in shares)		2,606,750	2,006,250
Carrying value		$ 26,068	$ 20,063
Share value and redemption price per share (in dollars per share)		$ 10,000
Series O
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)		125,750	125,750
Carrying value		$ 1,258	$ 1,258
Issue date		Aug. 27, 2012
Contractual rate in effect		5.50%
Series O | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date		Sep. 01, 2017
Series P
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)		90,000	90,000
Carrying value		$ 900	$ 900
Issue date		Feb. 05, 2013
Contractual rate in effect		5.45%
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)		92,500	92,500
Carrying value		$ 925	$ 925
Issue date		Jan. 30, 2014
Contractual rate in effect		6.70%
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)		88,000	88,000
Carrying value		$ 880	$ 880
Issue date		Jun. 23, 2014
Contractual rate in effect		6.30%
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
Carrying value		$ 1,430	$ 0
Issue date		Feb. 12, 2015
Contractual rate in effect		6.125%
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	0
Carrying value		$ 1,425	$ 0
Issue date		Jun. 04, 2015
Contractual rate in effect		6.10%
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	0
Carrying value		$ 1,150	$ 0
Issue date		Jul. 29, 2015
Contractual rate in effect		6.15%
Series BB | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date		Sep. 01, 2020
Series I
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares (in shares)		600,000	600,000
Carrying value		$ 6,000	$ 6,000
Issue date		Apr. 23, 2008
Contractual rate in effect		7.90%
Date at which dividend rate becomes floating		Apr. 30, 2018
Floating annual rate of three-month LIBOR plus		LIBOR + 3.47
Preferred stock dividend rate, variable, description of basis		three-month LIBOR
Preferred stock dividend rate, variable, basis spread		3.47%
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
Carrying value		$ 1,500	$ 1,500
Issue date		Apr. 23, 2013
Contractual rate in effect		5.15%
Date at which dividend rate becomes floating		May 01, 2023
Floating annual rate of three-month LIBOR plus		LIBOR + 3.25
Preferred stock dividend rate, variable, description of basis		three-month LIBOR
Preferred stock dividend rate, variable, basis spread		3.25%
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
Carrying value		$ 1,500	$ 1,500
Issue date		Jul. 29, 2013
Contractual rate in effect		6.00%
Date at which dividend rate becomes floating		Aug. 01, 2023
Floating annual rate of three-month LIBOR plus		LIBOR + 3.30
Preferred stock dividend rate, variable, description of basis		three-month LIBOR
Preferred stock dividend rate, variable, basis spread		3.30%
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
Carrying value		$ 2,000	$ 2,000
Issue date		Jan. 22, 2014
Contractual rate in effect		6.75%
Date at which dividend rate becomes floating		Feb. 01, 2024
Floating annual rate of three-month LIBOR plus		LIBOR + 3.78
Preferred stock dividend rate, variable, description of basis		three-month LIBOR
Preferred stock dividend rate, variable, basis spread		3.78%
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
Carrying value		$ 1,000	$ 1,000
Issue date		Mar. 10, 2014
Contractual rate in effect		6.125%
Date at which dividend rate becomes floating		Apr. 30, 2024
Floating annual rate of three-month LIBOR plus		LIBOR + 3.33
Preferred stock dividend rate, variable, description of basis		three-month LIBOR
Preferred stock dividend rate, variable, basis spread		3.33%
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
Carrying value		$ 2,500	$ 2,500
Issue date		Jun. 09, 2014
Contractual rate in effect		5.00%
Date at which dividend rate becomes floating		Jul. 01, 2019
Floating annual rate of three-month LIBOR plus		LIBOR + 3.32
Preferred stock dividend rate, variable, description of basis		three-month LIBOR
Preferred stock dividend rate, variable, basis spread		3.32%
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
Carrying value		$ 1,600	$ 1,600
Issue date		Sep. 23, 2014
Contractual rate in effect		6.10%
Date at which dividend rate becomes floating		Oct. 01, 2024
Floating annual rate of three-month LIBOR plus		LIBOR + 3.33
Preferred stock dividend rate, variable, description of basis		three-month LIBOR
Preferred stock dividend rate, variable, basis spread		3.33%
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	0
Carrying value		$ 2,000	$ 0
Issue date		Apr. 21, 2015
Contractual rate in effect		5.30%
Date at which dividend rate becomes floating		May 01, 2020
Floating annual rate of three-month LIBOR plus		LIBOR + 3.80
Preferred stock dividend rate, variable, description of basis		three-month LIBOR
Preferred stock dividend rate, variable, basis spread		3.80%
Series Z | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Earliest redemption date		May 01, 2020
Series J
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Contractual rate in effect	8.625%
Redemption Date	Sep. 01, 2013