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, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	$ (1)	$ (22)
Less than 12 Months, Fair Value	2,804	11,149
12 Months or Greater, Unrealized Losses	(52)	(80)
12 Months or Greater, Fair Value	7,518	9,168
Gross Unrealized Losses	(53)	(102)
Total Estimated Fair Value	10,322	20,317
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	0	(2)
Less than 12 Months, Fair Value	896	821
12 Months or Greater, Unrealized Losses	(9)	(27)
12 Months or Greater, Fair Value	1,383	3,240
Gross Unrealized Losses	(9)	(29)
Total Estimated Fair Value	2,279	4,061
U.S. government agencies securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	0	(1)
Less than 12 Months, Fair Value	30	206
12 Months or Greater, Unrealized Losses	(5)	(7)
12 Months or Greater, Fair Value	553	700
Gross Unrealized Losses	(5)	(8)
Total Estimated Fair Value	583	906
Non-U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	0	(1)
Less than 12 Months, Fair Value	86	203
12 Months or Greater, Unrealized Losses	(2)	(3)
12 Months or Greater, Fair Value	192	461
Gross Unrealized Losses	(2)	(4)
Total Estimated Fair Value	278	664
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	(1)	(14)
Less than 12 Months, Fair Value	1,600	7,674
12 Months or Greater, Unrealized Losses	(28)	(32)
12 Months or Greater, Fair Value	4,204	3,561
Gross Unrealized Losses	(29)	(46)
Total Estimated Fair Value	5,804	11,235
Residential mortgage and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Unrealized Losses	0	(4)
Less than 12 Months, Fair Value	192	2,245
12 Months or Greater, Unrealized Losses	(8)	(11)
12 Months or Greater, Fair Value	1,186	1,206
Gross Unrealized Losses	(8)	(15)
Total Estimated Fair Value	$ 1,378	$ 3,451