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	Jun. 30, 2019	Dec. 31, 2018
Available-for-Sale Debt Securities
Less than 12 Months, Unrealized Losses	$ (2)	$ (1)
Less than 12 Months, Fair Value	2,759	2,804
12 Months or Greater, Unrealized Losses	(5)	(52)
12 Months or Greater, Fair Value	3,449	7,518
Gross Unrealized Losses	(7)	(53)
Total Estimated Fair Value	6,208	10,322
U.S. treasury securities
Available-for-Sale Debt Securities
Less than 12 Months, Unrealized Losses	(1)	0
Less than 12 Months, Fair Value	1,401	896
12 Months or Greater, Unrealized Losses	(1)	(9)
12 Months or Greater, Fair Value	470	1,383
Gross Unrealized Losses	(2)	(9)
Total Estimated Fair Value	1,871	2,279
Certificates of deposit
Available-for-Sale Debt Securities
Less than 12 Months, Unrealized Losses	0
Less than 12 Months, Fair Value	8
12 Months or Greater, Unrealized Losses	0
12 Months or Greater, Fair Value	0
Gross Unrealized Losses	0
Total Estimated Fair Value	8
U.S. government agencies securities
Available-for-Sale Debt Securities
Less than 12 Months, Unrealized Losses	0	0
Less than 12 Months, Fair Value	391	30
12 Months or Greater, Unrealized Losses	0	(5)
12 Months or Greater, Fair Value	393	553
Gross Unrealized Losses	0	(5)
Total Estimated Fair Value	784	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	11	86
12 Months or Greater, Unrealized Losses	0	(2)
12 Months or Greater, Fair Value	161	192
Gross Unrealized Losses	0	(2)
Total Estimated Fair Value	172	278
Corporate debt securities
Available-for-Sale Debt Securities
Less than 12 Months, Unrealized Losses	(1)	(1)
Less than 12 Months, Fair Value	922	1,600
12 Months or Greater, Unrealized Losses	(3)	(28)
12 Months or Greater, Fair Value	1,988	4,204
Gross Unrealized Losses	(4)	(29)
Total Estimated Fair Value	2,910	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	26	192
12 Months or Greater, Unrealized Losses	(1)	(8)
12 Months or Greater, Fair Value	437	1,186
Gross Unrealized Losses	(1)	(8)
Total Estimated Fair Value	$ 463	$ 1,378