Investments and Cash - Fair Value of Available-for-Sale Municipal Bond Portfolio by State and Revenue Sources (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Revenue Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	$ 2,900	$ 3,285
Amortized Cost	2,679	3,172
Obligations of state and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities, available-for-sale, at fair value	495	508
Fixed Maturities [Member] | State General Obligation [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	396	448
Fixed Maturities [Member] | State General Obligation [Member] | New York [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	6	5
Fixed Maturities [Member] | State General Obligation [Member] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	23	19
Fixed Maturities [Member] | State General Obligation [Member] | California [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	68	63
Fixed Maturities [Member] | State General Obligation [Member] | Washington [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	52	80
Fixed Maturities [Member] | State General Obligation [Member] | Florida [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	8	8
Fixed Maturities [Member] | State General Obligation [Member] | Massachusetts [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	71	75
Fixed Maturities [Member] | State General Obligation [Member] | Illinois [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	18	16
Fixed Maturities [Member] | State General Obligation [Member] | Pennsylvania [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	38	35
Fixed Maturities [Member] | State General Obligation [Member] | District of Columbia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	30	41
Fixed Maturities [Member] | State General Obligation [Member] | Georgia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	11	10
Fixed Maturities [Member] | State General Obligation [Member] | All others [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	71	96
Fixed Maturities [Member] | Local General Obligation [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	549	670
Fixed Maturities [Member] | Local General Obligation [Member] | New York [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	46	49
Fixed Maturities [Member] | Local General Obligation [Member] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	122	170
Fixed Maturities [Member] | Local General Obligation [Member] | California [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	70	77
Fixed Maturities [Member] | Local General Obligation [Member] | Washington [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	69	81
Fixed Maturities [Member] | Local General Obligation [Member] | Florida [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	3	13
Fixed Maturities [Member] | Local General Obligation [Member] | Massachusetts [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	0	0
Fixed Maturities [Member] | Local General Obligation [Member] | Illinois [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	53	55
Fixed Maturities [Member] | Local General Obligation [Member] | Pennsylvania [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	4	5
Fixed Maturities [Member] | Local General Obligation [Member] | District of Columbia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	0	0
Fixed Maturities [Member] | Local General Obligation [Member] | Georgia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	10	10
Fixed Maturities [Member] | Local General Obligation [Member] | All others [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	172	210
Fixed Maturities [Member] | Revenue Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	2,900	3,285
Fixed Maturities [Member] | Revenue Bonds [Member] | New York [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	408	492
Fixed Maturities [Member] | Revenue Bonds [Member] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	287	344
Fixed Maturities [Member] | Revenue Bonds [Member] | California [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	380	378
Fixed Maturities [Member] | Revenue Bonds [Member] | Washington [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	181	193
Fixed Maturities [Member] | Revenue Bonds [Member] | Florida [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	233	220
Fixed Maturities [Member] | Revenue Bonds [Member] | Massachusetts [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	115	144
Fixed Maturities [Member] | Revenue Bonds [Member] | Illinois [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	125	127
Fixed Maturities [Member] | Revenue Bonds [Member] | Pennsylvania [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	95	98
Fixed Maturities [Member] | Revenue Bonds [Member] | District of Columbia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	69	92
Fixed Maturities [Member] | Revenue Bonds [Member] | Georgia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	92	94
Fixed Maturities [Member] | Revenue Bonds [Member] | All others [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	915	1,103
Fixed Maturities [Member] | Transportation [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	916	967
Amortized Cost	835	925
Fixed Maturities [Member] | Tax backed [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	426	471
Amortized Cost	397	458
Fixed Maturities [Member] | Higher education [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	488	557
Amortized Cost	456	543
Fixed Maturities [Member] | Water and Sewer [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	453	580
Amortized Cost	422	566
Fixed Maturities [Member] | Health Care [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	236	278
Amortized Cost	220	270
Fixed Maturities [Member] | Municipal utilities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	234	287
Amortized Cost	212	267
Fixed Maturities [Member] | All Others [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	147	145
Amortized Cost	137	143
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	3,845	4,403
Amortized Cost	3,564	4,272
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | New York [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	460	546
Amortized Cost	431	536
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	432	533
Amortized Cost	404	520
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | California [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	518	518
Amortized Cost	457	482
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Washington [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	302	354
Amortized Cost	284	349
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Florida [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	244	241
Amortized Cost	229	236
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Massachusetts [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	186	219
Amortized Cost	171	211
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Illinois [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	196	198
Amortized Cost	182	192
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Pennsylvania [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	137	138
Amortized Cost	128	136
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | District of Columbia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	99	133
Amortized Cost	94	131
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Georgia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	113	114
Amortized Cost	104	110
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | All others [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	1,158	1,409
Amortized Cost	$ 1,080	$ 1,369