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	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses		$ (1)
Less than 12 Months, Fair Value		2,804
12 Months or Greater, Unrealized Losses		(52)
12 Months or Greater, Fair Value		7,518
Gross Unrealized Losses		(53)
Total Estimated Fair Value		10,322
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses		0
Less than 12 Months, Fair Value		896
12 Months or Greater, Unrealized Losses		(9)
12 Months or Greater, Fair Value		1,383
Gross Unrealized Losses		(9)
Total Estimated Fair Value		2,279
U.S. government agencies securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses		0
Less than 12 Months, Fair Value		30
12 Months or Greater, Unrealized Losses		(5)
12 Months or Greater, Fair Value		553
Gross Unrealized Losses		(5)
Total Estimated Fair Value		583
Non-U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses		0
Less than 12 Months, Fair Value		86
12 Months or Greater, Unrealized Losses		(2)
12 Months or Greater, Fair Value		192
Gross Unrealized Losses		(2)
Total Estimated Fair Value		278
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	$ (1)	(1)
Less than 12 Months, Fair Value	1,866	1,600
12 Months or Greater, Unrealized Losses	0	(28)
12 Months or Greater, Fair Value	4	4,204
Gross Unrealized Losses	(1)	(29)
Total Estimated Fair Value	$ 1,870	5,804
Residential mortgage and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses		0
Less than 12 Months, Fair Value		192
12 Months or Greater, Unrealized Losses		(8)
12 Months or Greater, Fair Value		1,186
Gross Unrealized Losses		(8)
Total Estimated Fair Value		$ 1,378