Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities in a Continuous Unrealized Loss Position Deemed not to be Other-than-Temporarily Impaired (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Unrealized Losses	$ (35)	$ (22)
Less than 12 Months, Fair Value	8,812	11,149
12 Months or Greater, Unrealized Losses	(101)	(80)
12 Months or Greater, Fair Value	9,366	9,168
Gross Unrealized Losses	(136)	(102)
Total Estimated Fair Value	18,178	20,317
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Unrealized Losses	(25)	(14)
Less than 12 Months, Fair Value	6,256	7,674
12 Months or Greater, Unrealized Losses	(48)	(32)
12 Months or Greater, Fair Value	3,604	3,561
Gross Unrealized Losses	(73)	(46)
Total Estimated Fair Value	9,860	11,235
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Unrealized Losses	(2)	(2)
Less than 12 Months, Fair Value	547	821
12 Months or Greater, Unrealized Losses	(27)	(27)
12 Months or Greater, Fair Value	3,337	3,240
Gross Unrealized Losses	(29)	(29)
Total Estimated Fair Value	3,884	4,061
Residential mortgage and asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Unrealized Losses	(7)	(4)
Less than 12 Months, Fair Value	1,682	2,245
12 Months or Greater, Unrealized Losses	(15)	(11)
12 Months or Greater, Fair Value	1,289	1,206
Gross Unrealized Losses	(22)	(15)
Total Estimated Fair Value	2,971	3,451
U.S. government agencies securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Unrealized Losses	(1)	(1)
Less than 12 Months, Fair Value	182	206
12 Months or Greater, Unrealized Losses	(8)	(7)
12 Months or Greater, Fair Value	723	700
Gross Unrealized Losses	(9)	(8)
Total Estimated Fair Value	905	906
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Unrealized Losses	0	(1)
Less than 12 Months, Fair Value	145	203
12 Months or Greater, Unrealized Losses	(3)	(3)
12 Months or Greater, Fair Value	413	461
Gross Unrealized Losses	(3)	(4)
Total Estimated Fair Value	$ 558	$ 664

Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities in a Continuous Unrealized Loss Position Deemed not to be Other-than-Temporarily Impaired (Details) - USD ($)
$ in Millions

Mar. 31, 2018

Dec. 31, 2017

Schedule of Available-for-sale Securities [Line Items]

Less than 12 Months, Unrealized Losses

$ (35)

$ (22)

Less than 12 Months, Fair Value

8,812

11,149

12 Months or Greater, Unrealized Losses

(101)

(80)