Business Segments (Income Statement Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net written premiums	$ 7,656	$ 7,345	$ 7,069
Operating revenues
Net earned premiums	7,428	7,312	6,988
Net investment income	2,118	1,817	2,034
Non-insurance warranty revenue	1,161	1,007	390
Other revenues	31	50	37
Total operating revenues	10,738	10,186	9,449
Claims, Benefits and Expenses
Net incurred claims and benefits	5,783	5,547	5,288
Policyholders’ dividends	23	25	22
Amortization of deferred acquisition costs	1,383	1,335	1,233
Non-insurance warranty expense	1,082	923	299
Other insurance related expenses	1,038	1,039	1,098
Other expenses	235	301	292
Total claims, benefits and expenses	9,544	9,170	8,232
Core income (loss) before income tax	1,194	1,016	1,217
Income tax (expense) benefit on core income (loss)	(215)	(171)	(298)
Core income (loss)	979	845	919
Net investment gains (losses)	29	(52)	93
Income tax (expense) benefit on net investment gains (losses)	(8)	14	(30)
Net investment gains (losses), after tax	21	(38)	63
Net deferred tax asset remeasurement		6	(83)
Net income	1,000	813	899
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Net written premiums	2,848	2,744	2,731
Operating revenues
Net earned premiums	2,773	2,732	2,712
Net investment income	556	439	522
Non-insurance warranty revenue	1,161	1,007	390
Other revenues	1	2	1
Total operating revenues	4,491	4,180	3,625
Claims, Benefits and Expenses
Net incurred claims and benefits	1,595	1,526	1,533
Policyholders’ dividends	5	5	4
Amortization of deferred acquisition costs	610	599	590
Non-insurance warranty expense	1,082	923	299
Other insurance related expenses	292	279	279
Other expenses	48	46	43
Total claims, benefits and expenses	3,632	3,378	2,748
Core income (loss) before income tax	859	802	877
Income tax (expense) benefit on core income (loss)	(188)	(173)	(295)
Core income (loss)	671	629	582
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Net written premiums	3,315	3,060	2,922
Operating revenues
Net earned premiums	3,162	3,050	2,881
Net investment income	654	500	658
Non-insurance warranty revenue	0	0	0
Other revenues	29	28	32
Total operating revenues	3,845	3,578	3,571
Claims, Benefits and Expenses
Net incurred claims and benefits	2,130	2,053	1,930
Policyholders’ dividends	18	20	18
Amortization of deferred acquisition costs	537	505	481
Non-insurance warranty expense	0	0	0
Other insurance related expenses	505	505	530
Other expenses	32	43	57
Total claims, benefits and expenses	3,222	3,126	3,016
Core income (loss) before income tax	623	452	555
Income tax (expense) benefit on core income (loss)	(134)	(95)	(186)
Core income (loss)	489	357	369
Operating Segments | International
Segment Reporting Information [Line Items]
Net written premiums	971	1,018	881
Operating revenues
Net earned premiums	974	1,001	857
Net investment income	63	57	52
Non-insurance warranty revenue	0	0	0
Other revenues	0	1	0
Total operating revenues	1,037	1,059	909
Claims, Benefits and Expenses
Net incurred claims and benefits	624	699	575
Policyholders’ dividends	0	0	0
Amortization of deferred acquisition costs	236	231	162
Non-insurance warranty expense	0	0	0
Other insurance related expenses	130	135	162
Other expenses	8	14	(7)
Total claims, benefits and expenses	998	1,079	892
Core income (loss) before income tax	39	(20)	17
Income tax (expense) benefit on core income (loss)	(9)	1	(9)
Core income (loss)	30	(19)	8
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Net written premiums	523	524	536
Operating revenues
Net earned premiums	520	530	539
Net investment income	820	801	782
Non-insurance warranty revenue	0	0	0
Other revenues	0	2	2
Total operating revenues	1,340	1,333	1,323
Claims, Benefits and Expenses
Net incurred claims and benefits	1,416	1,218	1,269
Policyholders’ dividends	0	0	0
Amortization of deferred acquisition costs	0	0	0
Non-insurance warranty expense	0	0	0
Other insurance related expenses	115	122	129
Other expenses	8	7	7
Total claims, benefits and expenses	1,539	1,347	1,405
Core income (loss) before income tax	(199)	(14)	(82)
Income tax (expense) benefit on core income (loss)	90	57	132
Core income (loss)	(109)	43	50
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Net written premiums	1	0	0
Operating revenues
Net earned premiums	1	0	0
Net investment income	25	20	20
Non-insurance warranty revenue	0	0	0
Other revenues	6	19	2
Total operating revenues	32	39	22
Claims, Benefits and Expenses
Net incurred claims and benefits	18	51	(19)
Policyholders’ dividends	0	0	0
Amortization of deferred acquisition costs	0	0	0
Non-insurance warranty expense	0	0	0
Other insurance related expenses	(2)	(1)	(1)
Other expenses	144	193	192
Total claims, benefits and expenses	160	243	172
Core income (loss) before income tax	(128)	(204)	(150)
Income tax (expense) benefit on core income (loss)	26	39	60
Core income (loss)	(102)	(165)	(90)
Eliminations
Segment Reporting Information [Line Items]
Net written premiums	(2)	(1)	(1)
Operating revenues
Net earned premiums	(2)	(1)	(1)
Net investment income	0	0	0
Non-insurance warranty revenue	0	0	0
Other revenues	(5)	(2)	0
Total operating revenues	(7)	(3)	(1)
Claims, Benefits and Expenses
Net incurred claims and benefits	0	0	0
Policyholders’ dividends	0	0	0
Amortization of deferred acquisition costs	0	0	0
Non-insurance warranty expense	0	0	0
Other insurance related expenses	(2)	(1)	(1)
Other expenses	(5)	(2)	0
Total claims, benefits and expenses	(7)	(3)	(1)
Core income (loss) before income tax	0	0	0
Income tax (expense) benefit on core income (loss)	0	0	0
Core income (loss)	$ 0	$ 0	$ 0