Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
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	2,606,750
Carrying value	$ 26,068	$ 26,068
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	143,000
Carrying value	$ 1,430	$ 1,430
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	142,500
Carrying value	$ 1,425	$ 1,425
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	115,000
Carrying value	$ 1,150	$ 1,150
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	200,000
Carrying value	$ 2,000	$ 2,000
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