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	Sep. 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	$ (11)	$ (51)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	6,616	11,900
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1)
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	212	63
Available for Sale Securities - Gross Unrealized Losses	(12)	(51)
Continuous Unrealized Loss Position, Estimated Fair Value	6,828	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	(6)	(23)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	3,565	4,891
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	166	43
Available for Sale Securities - Gross Unrealized Losses	(6)	(23)
Continuous Unrealized Loss Position, Estimated Fair Value	3,731	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	(2)	(18)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	1,810	4,342
Available for Sale Securities - Gross Unrealized Losses	(2)	(18)
Continuous Unrealized Loss Position, Estimated Fair Value	1,810	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	389	1,626
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1)
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	35	20
Available for Sale Securities - Gross Unrealized Losses	(2)	(6)
Continuous Unrealized Loss Position, Estimated Fair Value	424	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	(1)	(2)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	553	707
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	0
Available for Sale Securities - Gross Unrealized Losses	(1)	(2)
Continuous Unrealized Loss Position, Estimated Fair Value	553	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	(1)	(2)
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	299	313
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	11	0
Available for Sale Securities - Gross Unrealized Losses	(1)	(2)
Continuous Unrealized Loss Position, Estimated Fair Value	$ 310	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