Derivative and Other Hedging Instruments (US Treasury Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
U.S. Treasury securities, net	$ (5,379)	$ (1,671)
7 Years Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(2,941)	(354)
At Par Value [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(5,311)	(1,689)
US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	(1)	4
TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	(14,881)	(7,295)
Derivative, Forward Settlement Value	(15,540)	(7,430)
Derivative Asset, Fair Value, Gross Asset	(15,586)	(7,444)
Derivative, Fair Value, Net	46	14
5 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(1,670)	(250)
10 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(700)	(1,085)
10 Year Maturity [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	0	2
5 Year Maturity [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	(1)	2
Short [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	(1,960)	(1,860)
Derivative, Forward Settlement Value	(2,498)	(2,290)
Derivative Asset, Fair Value, Gross Asset	(2,499)	(2,286)
Short [Member] | 10 Year Maturity [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	(1,230)	(1,130)
Derivative, Forward Settlement Value	(1,612)	(1,424)
Derivative Asset, Fair Value, Gross Asset	(1,612)	(1,422)
Short [Member] | 5 Year Maturity [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	(730)	(730)
Derivative, Forward Settlement Value	(886)	(866)
Derivative Asset, Fair Value, Gross Asset	(887)	(864)
Fair Value Hedging [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(5,379)	(1,671)
Fair Value Hedging [Member] | 7 Years Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(2,959)	(352)
Fair Value Hedging [Member] | 5 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(1,688)	(249)
Fair Value Hedging [Member] | 10 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(732)	(1,070)
At Cost Basis [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(5,310)	(1,680)
At Cost Basis [Member] | 7 Years Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(2,930)	(353)
At Cost Basis [Member] | 5 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(1,683)	(249)
At Cost Basis [Member] | 10 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	$ (697)	$ (1,078)