Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Jun. 30, 2020 USD ($) Security	Dec. 31, 2019 USD ($) Security
Less than 12 months
Fair value	$ 948	$ 145
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(22)	(1)
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	221	442
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(23)	(33)
Total
Unrealized loss position	1,169	587
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	$ (45)	$ (34)
Number of securities
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions | Security	196	57
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions | Security	65	119
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | Security	244	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	$ 35	$ 45
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(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	35	45
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(1)	(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	341	61
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(8)	0
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	61	119
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(12)	(19)
Total
Unrealized loss position	402	180
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(20)	(19)
RMBS [Member]
Less than 12 months
Fair value	30	10
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2)	0
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1	75
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(7)
Total
Unrealized loss position	31	85
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(2)	(7)
CMBS [Member]
Less than 12 months
Fair value	0	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	1	4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total
Unrealized loss position	1	4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	0	0
Asset-backed Securities [Member]
Less than 12 months
Fair value	468	24
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(10)	0
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	118	183
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(3)	(2)
Total
Unrealized loss position	586	207
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(13)	(2)
Foreign government securities [Member]
Less than 12 months
Fair value	74	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	0
12 months or more
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	40	56
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8)	(5)
Total
Unrealized loss position	114	56
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	$ (9)	$ (5)