Derivative and Other Hedging Instruments (US Treasury Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
U.S. Treasury securities, net	$ (7,454)	$ (1,671)
7 Years Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(3,056)	(354)
At Par Value [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(7,872)	(1,689)
US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	8	4
TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	(10,916)	(7,295)
Derivative, Forward Settlement Value	(11,312)	(7,430)
Derivative Asset, Fair Value, Gross Asset	(11,165)	(7,444)
Derivative, Fair Value, Net	(147)	14
5 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(400)	(250)
10 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(4,416)	(1,085)
10 Year Maturity [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	5	2
5 Year Maturity [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	3	2
Short [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	(1,810)	(1,860)
Derivative, Forward Settlement Value	(2,209)	(2,290)
Derivative Asset, Fair Value, Gross Asset	(2,201)	(2,286)
Short [Member] | 10 Year Maturity [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	(1,080)	(1,130)
Derivative, Forward Settlement Value	(1,347)	(1,424)
Derivative Asset, Fair Value, Gross Asset	(1,342)	(1,422)
Short [Member] | 5 Year Maturity [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	(730)	(730)
Derivative, Forward Settlement Value	(862)	(866)
Derivative Asset, Fair Value, Gross Asset	(859)	(864)
Fair Value Hedging [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(7,454)	(1,671)
Fair Value Hedging [Member] | 7 Years Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(2,930)	(352)
Fair Value Hedging [Member] | 5 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(392)	(249)
Fair Value Hedging [Member] | 10 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(4,132)	(1,070)
At Cost Basis [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(7,681)	(1,680)
At Cost Basis [Member] | 7 Years Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(3,041)	(353)
At Cost Basis [Member] | 5 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	(404)	(249)
At Cost Basis [Member] | 10 Year Maturity [Member]
Derivative [Line Items]
U.S. Treasury securities, net	$ (4,236)	$ (1,078)