Business Segments (Income Statement Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net written premiums	$ 7,345	$ 7,069	$ 6,988
Operating revenues
Net earned premiums	7,312	6,988	6,924
Net investment income	1,817	2,034	1,988
Non-insurance warranty revenue	1,007	390	361
Other revenues	50	37	43
Total operating revenues	10,186	9,449	9,316
Claims, Benefits and Expenses
Net incurred claims and benefits	5,547	5,288	5,270
Policyholders’ dividends	25	22	13
Amortization of deferred acquisition costs	1,335	1,233	1,235
Non-insurance warranty expense	923	299	271
Other insurance related expenses	1,039	1,098	1,122
Other expenses	301	292	318
Total claims, benefits and expenses	9,170	8,232	8,229
Core income (loss) before income tax	1,016	1,217	1,087
Income tax (expense) benefit on core income (loss)	(171)	(298)	(263)
Core income (loss)	845	919	824
Net realized investment gains (losses)	(52)	93	50
Income tax (expense) benefit on net realized investment gains (losses)	14	(30)	(15)
Net realized investment gains (losses), after tax	(38)	63	35
Net deferred tax asset remeasurement	6	(83)
Net income	813	899	859
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Net written premiums	2,744	2,731	2,738
Operating revenues
Net earned premiums	2,732	2,712	2,743
Net investment income	439	522	497
Non-insurance warranty revenue	1,007	390	361
Other revenues	2	1	1
Total operating revenues	4,180	3,625	3,602
Claims, Benefits and Expenses
Net incurred claims and benefits	1,526	1,533	1,489
Policyholders’ dividends	5	4	4
Amortization of deferred acquisition costs	599	590	586
Non-insurance warranty expense	923	299	271
Other insurance related expenses	279	279	295
Other expenses	46	43	41
Total claims, benefits and expenses	3,378	2,748	2,686
Core income (loss) before income tax	802	877	916
Income tax (expense) benefit on core income (loss)	(173)	(295)	(310)
Core income (loss)	629	582	606
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Net written premiums	3,060	2,922	2,883
Operating revenues
Net earned premiums	3,050	2,881	2,840
Net investment income	500	658	657
Non-insurance warranty revenue	0	0	0
Other revenues	28	32	32
Total operating revenues	3,578	3,571	3,529
Claims, Benefits and Expenses
Net incurred claims and benefits	2,053	1,930	1,905
Policyholders’ dividends	20	18	9
Amortization of deferred acquisition costs	505	481	475
Non-insurance warranty expense	0	0	0
Other insurance related expenses	505	530	566
Other expenses	43	57	36
Total claims, benefits and expenses	3,126	3,016	2,991
Core income (loss) before income tax	452	555	538
Income tax (expense) benefit on core income (loss)	(95)	(186)	(183)
Core income (loss)	357	369	355
Operating Segments | International
Segment Reporting Information [Line Items]
Net written premiums	1,018	881	821
Operating revenues
Net earned premiums	1,001	857	806
Net investment income	57	52	51
Non-insurance warranty revenue	0	0	0
Other revenues	1	0	0
Total operating revenues	1,059	909	857
Claims, Benefits and Expenses
Net incurred claims and benefits	699	575	492
Policyholders’ dividends	0	0	0
Amortization of deferred acquisition costs	231	162	174
Non-insurance warranty expense	0	0	0
Other insurance related expenses	135	162	133
Other expenses	14	(7)	24
Total claims, benefits and expenses	1,079	892	823
Core income (loss) before income tax	(20)	17	34
Income tax (expense) benefit on core income (loss)	1	(9)	(13)
Core income (loss)	(19)	8	21
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Net written premiums	524	536	547
Operating revenues
Net earned premiums	530	539	536
Net investment income	801	782	767
Non-insurance warranty revenue	0	0	0
Other revenues	2	2	(2)
Total operating revenues	1,333	1,323	1,301
Claims, Benefits and Expenses
Net incurred claims and benefits	1,218	1,269	1,286
Policyholders’ dividends	0	0	0
Amortization of deferred acquisition costs	0	0	0
Non-insurance warranty expense	0	0	0
Other insurance related expenses	122	129	132
Other expenses	7	7	8
Total claims, benefits and expenses	1,347	1,405	1,426
Core income (loss) before income tax	(14)	(82)	(125)
Income tax (expense) benefit on core income (loss)	57	132	145
Core income (loss)	43	50	20
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Net written premiums	0	0	0
Operating revenues
Net earned premiums	0	0	0
Net investment income	20	20	16
Non-insurance warranty revenue	0	0	0
Other revenues	19	2	12
Total operating revenues	39	22	28
Claims, Benefits and Expenses
Net incurred claims and benefits	51	(19)	98
Policyholders’ dividends	0	0	0
Amortization of deferred acquisition costs	0	0	0
Non-insurance warranty expense	0	0	0
Other insurance related expenses	(1)	(1)	(3)
Other expenses	193	192	209
Total claims, benefits and expenses	243	172	304
Core income (loss) before income tax	(204)	(150)	(276)
Income tax (expense) benefit on core income (loss)	39	60	98
Core income (loss)	(165)	(90)	(178)
Eliminations
Segment Reporting Information [Line Items]
Net written premiums	(1)	(1)	(1)
Operating revenues
Net earned premiums	(1)	(1)	(1)
Net investment income	0	0	0
Non-insurance warranty revenue	0	0	0
Other revenues	(2)	0	0
Total operating revenues	(3)	(1)	(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	(1)	(1)	(1)
Other expenses	(2)	0	0
Total claims, benefits and expenses	(3)	(1)	(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