Investments and Cash - Fair Value of Available-for-Sale Municipal Bond Portfolio by State and Revenue Sources (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Revenue Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	$ 3,546	$ 3,370
Amortized Cost	3,369	3,271
Obligations of state and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities, available-for-sale, at fair value	892	966
Fixed Maturities [Member] | State General Obligation [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	522	482
Fixed Maturities [Member] | State General Obligation [Member] | New York [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	13	13
Fixed Maturities [Member] | State General Obligation [Member] | California [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	76	73
Fixed Maturities [Member] | State General Obligation [Member] | Texas [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	17	16
Fixed Maturities [Member] | State General Obligation [Member] | Washington [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	93	81
Fixed Maturities [Member] | State General Obligation [Member] | Florida [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	5	16
Fixed Maturities [Member] | State General Obligation [Member] | Massachusetts [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	70	74
Fixed Maturities [Member] | State General Obligation [Member] | Illinois [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	18	18
Fixed Maturities [Member] | State General Obligation [Member] | Ohio [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	16
Fixed Maturities [Member] | State General Obligation [Member] | Pennsylvania [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	33	38
Fixed Maturities [Member] | State General Obligation [Member] | District of Columbia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	43
Fixed Maturities [Member] | State General Obligation [Member] | Arizona [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities		0
Fixed Maturities [Member] | State General Obligation [Member] | Georgia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities		0
Fixed Maturities [Member] | State General Obligation [Member] | All others [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	138	153
Fixed Maturities [Member] | Local General Obligation [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	800	614
Fixed Maturities [Member] | Local General Obligation [Member] | New York [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	44	38
Fixed Maturities [Member] | Local General Obligation [Member] | California [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	83	62
Fixed Maturities [Member] | Local General Obligation [Member] | Texas [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	212	186
Fixed Maturities [Member] | Local General Obligation [Member] | Washington [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	87	68
Fixed Maturities [Member] | Local General Obligation [Member] | Florida [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	17	11
Fixed Maturities [Member] | Local General Obligation [Member] | Massachusetts [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	0	0
Fixed Maturities [Member] | Local General Obligation [Member] | Illinois [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	51	65
Fixed Maturities [Member] | Local General Obligation [Member] | Ohio [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	22
Fixed Maturities [Member] | Local General Obligation [Member] | Pennsylvania [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	21	17
Fixed Maturities [Member] | Local General Obligation [Member] | District of Columbia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	0
Fixed Maturities [Member] | Local General Obligation [Member] | Arizona [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities		3
Fixed Maturities [Member] | Local General Obligation [Member] | Georgia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities		9
Fixed Maturities [Member] | Local General Obligation [Member] | All others [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	263	155
Fixed Maturities [Member] | Revenue Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	3,546	3,370
Fixed Maturities [Member] | Revenue Bonds [Member] | New York [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	568	570
Fixed Maturities [Member] | Revenue Bonds [Member] | California [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	421	391
Fixed Maturities [Member] | Revenue Bonds [Member] | Texas [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	321	316
Fixed Maturities [Member] | Revenue Bonds [Member] | Washington [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	214	201
Fixed Maturities [Member] | Revenue Bonds [Member] | Florida [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	244	247
Fixed Maturities [Member] | Revenue Bonds [Member] | Massachusetts [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	151	149
Fixed Maturities [Member] | Revenue Bonds [Member] | Illinois [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	131	127
Fixed Maturities [Member] | Revenue Bonds [Member] | Ohio [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	102
Fixed Maturities [Member] | Revenue Bonds [Member] | Pennsylvania [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	76	58
Fixed Maturities [Member] | Revenue Bonds [Member] | District of Columbia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	85
Fixed Maturities [Member] | Revenue Bonds [Member] | Arizona [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities		122
Fixed Maturities [Member] | Revenue Bonds [Member] | Georgia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities		104
Fixed Maturities [Member] | Revenue Bonds [Member] | All others [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	1,233	1,085
Fixed Maturities [Member] | Transportation [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	955	860
Amortized Cost	889	824
Fixed Maturities [Member] | Water and Sewer [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	670	545
Amortized Cost	641	531
Fixed Maturities [Member] | Tax backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	600	617
Amortized Cost	570	601
Fixed Maturities [Member] | Higher education [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	515	513
Amortized Cost	492	499
Fixed Maturities [Member] | Municipal utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	324	365
Amortized Cost	315	360
Fixed Maturities [Member] | Healthcare [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	308	310
Amortized Cost	293	298
Fixed Maturities [Member] | All Others [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	174	160
Amortized Cost	169	158
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	4,868	4,466
Amortized Cost	4,646	4,344
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | New York [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	625	621
Amortized Cost	598	604
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | California [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	580	526
Amortized Cost	527	497
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Texas [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	550	518
Amortized Cost	528	503
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Washington [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	394	350
Amortized Cost	381	348
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Florida [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	266	274
Amortized Cost	254	266
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Massachusetts [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	221	223
Amortized Cost	208	215
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Illinois [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	200	210
Amortized Cost	189	205
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Ohio [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	140
Amortized Cost	136
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Pennsylvania [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	130	113
Amortized Cost	125	111
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | District of Columbia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	128
Amortized Cost	123
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Arizona [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities		125
Amortized Cost		122
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | Georgia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities		113
Amortized Cost		109
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member] | All others [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	1,634	1,393
Amortized Cost	$ 1,577	$ 1,364