Investments - Unrealized Losses and Fair Values (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values		$ 6,838
Less than 12 months, Unrealized losses		(226)
12 months or more, Fair values		4,360
12 months or more, Unrealized losses		(115)
Total realized losses	$ 36	394	$ 213
U.S. Treasury notes [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values		1,219
Less than 12 months, Unrealized losses		(21)
12 months or more, Fair values		1,444
12 months or more, Unrealized losses		(26)
Other government-related debt securities - U.S. [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values		0
Less than 12 months, Unrealized losses		0
12 months or more, Fair values		110
12 months or more, Unrealized losses		(2)
Other government-related debt securities - Foreign and other [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values		631
Less than 12 months, Unrealized losses		(31)
12 months or more, Fair values		240
12 months or more, Unrealized losses		(10)
Corporate debt securities - Financial [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values		1,968
Less than 12 months, Unrealized losses		(59)
12 months or more, Fair values		718
12 months or more, Unrealized losses		(22)
Corporate debt securities - Industrial [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values		1,898
Less than 12 months, Unrealized losses		(81)
12 months or more, Fair values		529
12 months or more, Unrealized losses		(18)
Corporate debt securities - Other [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values		529
Less than 12 months, Unrealized losses		(20)
12 months or more, Fair values		28
12 months or more, Unrealized losses		(1)
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values		576
Less than 12 months, Unrealized losses		(14)
12 months or more, Fair values		840
12 months or more, Unrealized losses		(22)
Other mortgage- and asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values		17
Less than 12 months, Unrealized losses		0
12 months or more, Fair values		451
12 months or more, Unrealized losses		$ (14)