Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Mar. 31, 2020 USD ($) Security	Dec. 31, 2019 USD ($) Security
Less than 12 months
Fair value	$ 1,390	$ 145
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(62)	(1)
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	260	442
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(30)	(33)
Total
Unrealized loss position	1,650	587
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	$ (92)	$ (34)
Number of securities
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | Security	376	57
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions | Security	79	119
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | Security	430	176
Number of securities with OTTI
Less than 12 months (in securities) | Security		1
12 months or more (in securities) | Security		7
Total (in securities) | Security		8
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value	$ 104	$ 45
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4)	(1)
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total
Unrealized loss position	104	45
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(4)	(1)
US government and agencies [Member]
Less than 12 months
Fair value		5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Total
Unrealized loss position		10
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		0
Corporate securities [Member]
Less than 12 months
Fair value	679	61
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(29)	0
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	68	119
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(13)	(19)
Total
Unrealized loss position	747	180
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(42)	(19)
RMBS [Member]
Less than 12 months
Fair value	42	10
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4)	0
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	5	75
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(7)
Total
Unrealized loss position	47	85
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(4)	(7)
CMBS [Member]
Less than 12 months
Fair value	36	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3	4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total
Unrealized loss position	39	4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	0	0
Asset-backed Securities [Member]
Less than 12 months
Fair value	434	24
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(22)	0
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	132	183
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8)	(2)
Total
Unrealized loss position	566	207
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(30)	(2)
Foreign government securities [Member]
Less than 12 months
Fair value	95	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3)	0
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	52	56
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(9)	(5)
Total
Unrealized loss position	147	56
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	$ (12)	$ (5)