Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Sep. 30, 2020 USD ($) Security	Dec. 31, 2019 USD ($) Security
Less than 12 months
Fair value	$ 300	$ 145
Unrealized Loss	$ (4)	$ (1)
Number of securities | Security	93	57
Number of securities with OTTI | Security		1
12 months or more
Fair Value	$ 194	$ 442
Unrealized Loss	$ (16)	$ (33)
Number of securities | Security	55	119
Number of securities with OTTI | Security		7
Total
Fair Value	$ 494	$ 587
Unrealized Loss	$ (20)	$ (34)
Number of securities | Security	147	176
Number of securities with OTTI | Security		8
Obligations of state and political subdivisions
Less than 12 months
Fair value	$ 89	$ 45
Unrealized Loss	(2)	(1)
12 months or more
Fair Value	0	0
Unrealized Loss	0	0
Total
Fair Value	89	45
Unrealized Loss	(2)	(1)
U.S. government and agencies
Less than 12 months
Fair value	20	5
Unrealized Loss	0	0
12 months or more
Fair Value	0	5
Unrealized Loss	0	0
Total
Fair Value	20	10
Unrealized Loss	0	0
Corporate securities
Less than 12 months
Fair value	39	61
Unrealized Loss	(1)	0
12 months or more
Fair Value	57	119
Unrealized Loss	(8)	(19)
Total
Fair Value	96	180
Unrealized Loss	(9)	(19)
RMBS
Less than 12 months
Fair value	20	10
Unrealized Loss	(1)	0
12 months or more
Fair Value	1	75
Unrealized Loss	0	(7)
Total
Fair Value	21	85
Unrealized Loss	(1)	(7)
Commercial mortgage-backed securities (CMBS)
Less than 12 months
Fair value	0	0
Unrealized Loss	0	0
12 months or more
Fair Value	1	4
Unrealized Loss	0	0
Total
Fair Value	1	4
Unrealized Loss	0	0
Asset-backed securities
Less than 12 months
Fair value	122	24
Unrealized Loss	0	0
12 months or more
Fair Value	94	183
Unrealized Loss	(1)	(2)
Total
Fair Value	216	207
Unrealized Loss	(1)	(2)
Non-U.S. government securities
Less than 12 months
Fair value	10	0
Unrealized Loss	0	0
12 months or more
Fair Value	41	56
Unrealized Loss	(7)	(5)
Total
Fair Value	51	56
Unrealized Loss	$ (7)	$ (5)