Investments and Cash - Fair Value of Available-for-Sale Municipal Bond Portfolio by State and Revenue Sources (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Revenue Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	$ 3,285	$ 3,546
Amortized Cost	3,172	3,369
Obligations of state and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities, available-for-sale, at fair value	508	892
Fixed Maturities [Member] | State General Obligation [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	448	522
Fixed Maturities [Member] | State General Obligation [Member] | New York [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	5	13
Fixed Maturities [Member] | State General Obligation [Member] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	19	17
Fixed Maturities [Member] | State General Obligation [Member] | California [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	63	76
Fixed Maturities [Member] | State General Obligation [Member] | Washington [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	80	93
Fixed Maturities [Member] | State General Obligation [Member] | Florida [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	8	5
Fixed Maturities [Member] | State General Obligation [Member] | Massachusetts [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	75	70
Fixed Maturities [Member] | State General Obligation [Member] | Illinois [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	16	18
Fixed Maturities [Member] | State General Obligation [Member] | Pennsylvania [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	35	33
Fixed Maturities [Member] | State General Obligation [Member] | District of Columbia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	41	43
Fixed Maturities [Member] | State General Obligation [Member] | Georgia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	10
Fixed Maturities [Member] | State General Obligation [Member] | Ohio [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities		16
Fixed Maturities [Member] | State General Obligation [Member] | All others [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	96	138
Fixed Maturities [Member] | Local General Obligation [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	670	800
Fixed Maturities [Member] | Local General Obligation [Member] | New York [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	49	44
Fixed Maturities [Member] | Local General Obligation [Member] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	170	212
Fixed Maturities [Member] | Local General Obligation [Member] | California [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	77	83
Fixed Maturities [Member] | Local General Obligation [Member] | Washington [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	81	87
Fixed Maturities [Member] | Local General Obligation [Member] | Florida [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	13	17
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	55	51
Fixed Maturities [Member] | Local General Obligation [Member] | Pennsylvania [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	5	21
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
Fixed Maturities [Member] | Local General Obligation [Member] | Ohio [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities		22
Fixed Maturities [Member] | Local General Obligation [Member] | All others [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	210	263
Fixed Maturities [Member] | Revenue Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	3,285	3,546
Fixed Maturities [Member] | Revenue Bonds [Member] | New York [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	492	568
Fixed Maturities [Member] | Revenue Bonds [Member] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	344	321
Fixed Maturities [Member] | Revenue Bonds [Member] | California [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	378	421
Fixed Maturities [Member] | Revenue Bonds [Member] | Washington [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	193	214
Fixed Maturities [Member] | Revenue Bonds [Member] | Florida [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	220	244
Fixed Maturities [Member] | Revenue Bonds [Member] | Massachusetts [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	144	151
Fixed Maturities [Member] | Revenue Bonds [Member] | Illinois [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	127	131
Fixed Maturities [Member] | Revenue Bonds [Member] | Pennsylvania [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	98	76
Fixed Maturities [Member] | Revenue Bonds [Member] | District of Columbia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	92	85
Fixed Maturities [Member] | Revenue Bonds [Member] | Georgia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	94
Fixed Maturities [Member] | Revenue Bonds [Member] | Ohio [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities		102
Fixed Maturities [Member] | Revenue Bonds [Member] | All others [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	1,103	1,233
Fixed Maturities [Member] | Transportation [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	967	955
Amortized Cost	925	889
Fixed Maturities [Member] | Water and Sewer [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	580	670
Amortized Cost	566	641
Fixed Maturities [Member] | Tax backed [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	471	600
Amortized Cost	458	570
Fixed Maturities [Member] | Higher education [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	557	515
Amortized Cost	543	492
Fixed Maturities [Member] | Municipal utilities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	287	324
Amortized Cost	267	315
Fixed Maturities [Member] | Health Care [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	278	308
Amortized Cost	270	293
Fixed Maturities [Member] | All Others [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	145	174
Amortized Cost	143	169
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	4,403	4,868
Amortized Cost	4,272	4,646
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | New York [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	546	625
Amortized Cost	536	598
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	533	550
Amortized Cost	520	528
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | California [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	518	580
Amortized Cost	482	527
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Washington [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	354	394
Amortized Cost	349	381
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Florida [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	241	266
Amortized Cost	236	254
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Massachusetts [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	219	221
Amortized Cost	211	208
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Illinois [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	198	200
Amortized Cost	192	189
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Pennsylvania [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	138	130
Amortized Cost	136	125
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | District of Columbia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	133	128
Amortized Cost	131	123
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Georgia [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	114
Amortized Cost	110
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Ohio [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities		140
Amortized Cost		136
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | All others [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed-maturity securities	1,409	1,634
Amortized Cost	$ 1,369	$ 1,577