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, 2019	Dec. 31, 2018
Available-for-Sale Debt Securities
Less than 12 Months, Unrealized Losses	$ (1)	$ (1)
Less than 12 Months, Fair Value	4,688	2,804
12 Months or Greater, Unrealized Losses	(22)	(52)
12 Months or Greater, Fair Value	5,290	7,518
Gross Unrealized Losses	(23)	(53)
Total Estimated Fair Value	9,978	10,322
U.S. treasury securities
Available-for-Sale Debt Securities
Less than 12 Months, Unrealized Losses	0	0
Less than 12 Months, Fair Value	2,971	896
12 Months or Greater, Unrealized Losses	(4)	(9)
12 Months or Greater, Fair Value	917	1,383
Gross Unrealized Losses	(4)	(9)
Total Estimated Fair Value	3,888	2,279
U.S. government agencies securities
Available-for-Sale Debt Securities
Less than 12 Months, Unrealized Losses	0	0
Less than 12 Months, Fair Value	483	30
12 Months or Greater, Unrealized Losses	(2)	(5)
12 Months or Greater, Fair Value	458	553
Gross Unrealized Losses	(2)	(5)
Total Estimated Fair Value	941	583
Non-U.S. government securities
Available-for-Sale Debt Securities
Less than 12 Months, Unrealized Losses	0	0
Less than 12 Months, Fair Value	100	86
12 Months or Greater, Unrealized Losses	(1)	(2)
12 Months or Greater, Fair Value	160	192
Gross Unrealized Losses	(1)	(2)
Total Estimated Fair Value	260	278
Corporate debt securities
Available-for-Sale Debt Securities
Less than 12 Months, Unrealized Losses	(1)	(1)
Less than 12 Months, Fair Value	1,049	1,600
12 Months or Greater, Unrealized Losses	(11)	(28)
12 Months or Greater, Fair Value	2,938	4,204
Gross Unrealized Losses	(12)	(29)
Total Estimated Fair Value	3,987	5,804
Residential mortgage and asset-backed securities
Available-for-Sale Debt Securities
Less than 12 Months, Unrealized Losses	0	0
Less than 12 Months, Fair Value	85	192
12 Months or Greater, Unrealized Losses	(4)	(8)
12 Months or Greater, Fair Value	817	1,186
Gross Unrealized Losses	(4)	(8)
Total Estimated Fair Value	$ 902	$ 1,378