Investments - Securities in a Continuous Unrealized Loss Position With No Allowance for Credit Losses (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Fair Value
Less than 12 Months	$ 440	$ 160
12 Months or Longer	65	84
Fair Value, Total	505	244
Unrealized Losses
Less than 12 Months	(11)	(3)
12 Months or Longer	(6)	(3)
Unrealized Loss, Total	(17)	(6)
U.S. government and agencies
Fair Value
Less than 12 Months		62
12 Months or Longer		0
Fair Value, Total		62
Unrealized Losses
Less than 12 Months		(1)
12 Months or Longer		0
Unrealized Loss, Total		(1)
Corporate debt securities
Fair Value
Less than 12 Months		98
12 Months or Longer		51
Fair Value, Total		149
Unrealized Losses
Less than 12 Months		(2)
12 Months or Longer		(1)
Unrealized Loss, Total		(3)
Corporate debt securities | No Allowance For Credit Losses
Fair Value
Less than 12 Months	404
12 Months or Longer	45
Fair Value, Total	449
Unrealized Losses
Less than 12 Months	(9)
12 Months or Longer	(2)
Unrealized Loss, Total	(11)
Foreign government bonds
Fair Value
Less than 12 Months		0
12 Months or Longer		33
Fair Value, Total		33
Unrealized Losses
Less than 12 Months		0
12 Months or Longer		(2)
Unrealized Loss, Total		$ (2)
Foreign government bonds | No Allowance For Credit Losses
Fair Value
Less than 12 Months	36
12 Months or Longer	20
Fair Value, Total	56
Unrealized Losses
Less than 12 Months	(2)
12 Months or Longer	(4)
Unrealized Loss, Total	$ (6)