Preferred Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 01, 2013	Feb. 12, 2015
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares of preferred stock authorized to issue, in one or more series	200,000,000	200,000,000
Preferred stock, par value (in dollars per share)	$ 1.00	$ 1.00
Shares	2,006,250	1,115,750
Carrying value (in millions)	$ 20,063	$ 11,158
Share value and redemption price per share	$ 10,000
Preferred stock
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Issuance of preferred stock	8,905	3,900	1,258
Series O
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares	125,750	125,750
Carrying value (in millions)	1,258	1,258
Preferred Stock Issue Date	Aug. 27, 2012
Preferred stock dividend rate	5.50%
Series O | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock, Earliest Redemption Date	Sep. 01, 2017
Series P
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares	90,000	90,000
Carrying value (in millions)	900	900
Preferred Stock Issue Date	Feb. 05, 2013
Preferred stock dividend rate	5.45%
Series P | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock, Earliest Redemption Date	Mar. 01, 2018
Series T
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares	92,500	0
Carrying value (in millions)	925	0
Preferred Stock Issue Date	Jan. 30, 2014
Preferred stock dividend rate	6.70%
Series T | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock, Earliest Redemption Date	Mar. 01, 2019
Series W
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares	88,000	0
Carrying value (in millions)	880	0
Preferred Stock Issue Date	Jun. 23, 2014
Preferred stock dividend rate	6.30%
Series W | Minimum
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock, Earliest Redemption Date	Sep. 01, 2019
Series I
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares	600,000	600,000
Carrying value (in millions)	6,000	6,000
Preferred Stock Issue Date	Apr. 23, 2008
Preferred stock dividend rate	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]
Preferred Stock, Earliest Redemption Date	Apr. 30, 2018
Series Q
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares	150,000	150,000
Carrying value (in millions)	1,500	1,500
Preferred Stock Issue Date	Apr. 23, 2013
Preferred stock dividend rate	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]
Preferred Stock, Earliest Redemption Date	May 01, 2023
Series R
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares	150,000	150,000
Carrying value (in millions)	1,500	1,500
Preferred Stock Issue Date	Jul. 29, 2013
Preferred stock dividend rate	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]
Preferred Stock, Earliest Redemption Date	Aug. 01, 2023
Series S
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares	200,000	0
Carrying value (in millions)	2,000	0
Preferred Stock Issue Date	Jan. 22, 2014
Preferred stock dividend rate	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]
Preferred Stock, Earliest Redemption Date	Feb. 01, 2024
Series U
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares	100,000	0
Carrying value (in millions)	1,000	0
Preferred Stock Issue Date	Mar. 10, 2014
Preferred stock dividend rate	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]
Preferred Stock, Earliest Redemption Date	Apr. 30, 2024
Series V
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares	250,000	0
Carrying value (in millions)	2,500	0
Preferred Stock Issue Date	Jun. 09, 2014
Preferred stock dividend rate	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]
Preferred Stock, Earliest Redemption Date	Jul. 01, 2019
Series X
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares	160,000	0
Carrying value (in millions)	1,600	0
Preferred Stock Issue Date	Sep. 23, 2014
Preferred stock dividend rate	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]
Preferred Stock, Earliest Redemption Date	Oct. 01, 2024
Series J
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock dividend rate				8.625%
Preferred Stock, Redemption Date				Sep. 01, 2013
Series Y | Preferred stock | Subsequent Event
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock Issue Date					Feb. 12, 2015
Issuance of preferred stock					$ 1,400