Available-for-Sale Securities - Summary of Available-for-Sale Securities in a Continuous Unrealized Loss Position Deemed not to be Other-than-Temporarily Impaired (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ (3)	$ (51)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	1,448	11,900
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	143	63
Available for Sale Securities - Gross Unrealized Losses	(3)	(51)
Continuous Unrealized Loss Position, Estimated Fair Value	1,591	11,963
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2)	(23)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	833	4,891
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	124	43
Available for Sale Securities - Gross Unrealized Losses	(2)	(23)
Continuous Unrealized Loss Position, Estimated Fair Value	957	4,934
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(18)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	16	4,342
Available for Sale Securities - Gross Unrealized Losses	0	(18)
Continuous Unrealized Loss Position, Estimated Fair Value	16	4,342
Residential mortgage and asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(6)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	478	1,626
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	12	20
Available for Sale Securities - Gross Unrealized Losses	(1)	(6)
Continuous Unrealized Loss Position, Estimated Fair Value	490	1,646
U.S. government agencies securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(2)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	90	707
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	0
Available for Sale Securities - Gross Unrealized Losses	0	(2)
Continuous Unrealized Loss Position, Estimated Fair Value	90	707
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(2)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	31	313
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	7	0
Available for Sale Securities - Gross Unrealized Losses		(2)
Continuous Unrealized Loss Position, Estimated Fair Value	$ 38	313
Municipal debt securities
Schedule of Available-for-sale Securities [Line Items]
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value		21
Continuous Unrealized Loss Position, Estimated Fair Value		$ 21