Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Dec. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Less than 12 months
Fair value	$ 1,643	$ 806
Unrealized loss	(34)	(10)
12 months or more
Fair Value	1,952	975
Unrealized loss	(105)	(57)
Total
Fair value	3,595	1,781
Unrealized loss	$ (139)	$ (67)
Number of securities
Less than 12 months (in securities) | Security	417	244
12 months or more (in securities) | Security	608	264
Total (in securities) | Security	997	499
Number of securities with OTTI
Less than 12 months (in securities) | Security	22	17
12 months or more (in securities) | Security	22	15
Total (in securities) | Security	42	31
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value	$ 195	$ 166
Unrealized loss	(4)	(4)
12 months or more
Fair Value	658	281
Unrealized loss	(14)	(7)
Total
Fair value	853	447
Unrealized loss	(18)	(11)
US Treasury and Government [Member]
Less than 12 months
Fair value	11	151
Unrealized loss	0	0
12 months or more
Fair Value	24	18
Unrealized loss	(1)	(1)
Total
Fair value	35	169
Unrealized loss	(1)	(1)
Corporate securities [Member]
Less than 12 months
Fair value	836	201
Unrealized loss	(19)	(1)
12 months or more
Fair Value	522	240
Unrealized loss	(33)	(17)
Total
Fair value	1,358	441
Unrealized loss	(52)	(18)
Residential Mortgage-Backed Securities (RMBS) [Member]
Less than 12 months
Fair value	85	191
Unrealized loss	(2)	(5)
12 months or more
Fair Value	447	213
Unrealized loss	(32)	(12)
Total
Fair value	532	404
Unrealized loss	(34)	(17)
Commercial Mortgage Backed Securities (CMBS) [Member]
Less than 12 months
Fair value	111	29
Unrealized loss	(1)	0
12 months or more
Fair Value	164	80
Unrealized loss	(6)	(3)
Total
Fair value	275	109
Unrealized loss	(7)	(3)
Asset-backed Securities [Member]
Less than 12 months
Fair value	322	48
Unrealized loss	(4)	0
12 months or more
Fair Value	38	3
Unrealized loss	(1)	0
Total
Fair value	360	51
Unrealized loss	(5)	0
Foreign government securities [Member]
Less than 12 months
Fair value	83	20
Unrealized loss	(4)	0
12 months or more
Fair Value	99	140
Unrealized loss	(18)	(17)
Total
Fair value	182	160
Unrealized loss	$ (22)	$ (17)