Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Dec. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Less than 12 months
Fair value	$ 145	$ 1,643
Unrealized loss	(1)	(34)
12 months or more
Fair Value	442	1,952
Unrealized loss	(33)	(105)
Total
Fair value	587	3,595
Unrealized loss	$ (34)	$ (139)
Number of securities
Less than 12 months (in securities) | Security	57	417
12 months or more (in securities) | Security	119	608
Total (in securities) | Security	176	997
Number of securities with OTTI
Less than 12 months (in securities) | Security	1	22
12 months or more (in securities) | Security	7	22
Total (in securities) | Security	8	42
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value	$ 45	$ 195
Unrealized loss	(1)	(4)
12 months or more
Fair Value	0	658
Unrealized loss	0	(14)
Total
Fair value	45	853
Unrealized loss	(1)	(18)
US Treasury and Government [Member]
Less than 12 months
Fair value	5	11
Unrealized loss	0	0
12 months or more
Fair Value	5	24
Unrealized loss	0	(1)
Total
Fair value	10	35
Unrealized loss	0	(1)
Corporate securities [Member]
Less than 12 months
Fair value	61	836
Unrealized loss	0	(19)
12 months or more
Fair Value	119	522
Unrealized loss	(19)	(33)
Total
Fair value	180	1,358
Unrealized loss	(19)	(52)
Residential Mortgage-Backed Securities (RMBS) [Member]
Less than 12 months
Fair value	10	85
Unrealized loss	0	(2)
12 months or more
Fair Value	75	447
Unrealized loss	(7)	(32)
Total
Fair value	85	532
Unrealized loss	(7)	(34)
Commercial Mortgage Backed Securities (CMBS) [Member]
Less than 12 months
Fair value	0	111
Unrealized loss	0	(1)
12 months or more
Fair Value	4	164
Unrealized loss	0	(6)
Total
Fair value	4	275
Unrealized loss	0	(7)
Asset-backed Securities [Member]
Less than 12 months
Fair value	24	322
Unrealized loss	0	(4)
12 months or more
Fair Value	183	38
Unrealized loss	(2)	(1)
Total
Fair value	207	360
Unrealized loss	(2)	(5)
Foreign government securities [Member]
Less than 12 months
Fair value	0	83
Unrealized loss	0	(4)
12 months or more
Fair Value	56	99
Unrealized loss	(5)	(18)
Total
Fair value	56	182
Unrealized loss	$ (5)	$ (22)