Business Segments (Income Statement Information) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating revenues
Net earned premiums	$ 1,853	$ 1,806	$ 5,453	$ 5,185
Net investment income	487	509	1,483	1,529
Non-insurance warranty revenue	258	99	744	290
Other revenues	10	8	26	28
Total operating revenues	2,608	2,422	7,706	7,032
Claims, Benefits and Expenses
Net incurred claims and benefits	1,306	1,475	3,960	4,039
Policyholders’ dividends	6	5	18	14
Amortization of deferred acquisition costs	337	309	992	926
Non-insurance warranty expense	235	74	676	216
Other insurance related expenses	266	281	779	796
Other expenses	70	67	228	203
Total claims, benefits and expenses	2,220	2,211	6,653	6,194
Core income (loss) before income tax	388	211	1,053	838
Income tax (expense) benefit on core income (loss)	(71)	(52)	(185)	(205)
Core income (loss)	317	159	868	633
Net realized investment gains (losses)	14	(24)	25	62
Income tax (expense) benefit on net realized investment gains (losses)	(1)	9	(2)	(19)
Net realized investment gains (losses), after tax	13	(15)	23	43
Net deferred tax asset remeasurement	6		6
Net income	336	144	897	676
Operating Segments | Specialty
Operating revenues
Net earned premiums	684	692	2,039	2,024
Net investment income	124	129	376	394
Non-insurance warranty revenue	258	99	744	290
Other revenues	0	1	1	2
Total operating revenues	1,066	921	3,160	2,710
Claims, Benefits and Expenses
Net incurred claims and benefits	373	358	1,124	1,152
Policyholders’ dividends	1	1	3	3
Amortization of deferred acquisition costs	153	151	447	440
Non-insurance warranty expense	235	74	676	216
Other insurance related expenses	68	65	202	203
Other expenses	11	11	34	32
Total claims, benefits and expenses	841	660	2,486	2,046
Core income (loss) before income tax	225	261	674	664
Income tax (expense) benefit on core income (loss)	(48)	(88)	(143)	(223)
Core income (loss)	177	173	531	441
Operating Segments | Commercial
Operating revenues
Net earned premiums	782	752	2,278	2,129
Net investment income	144	166	450	495
Non-insurance warranty revenue	0	0	0	0
Other revenues	8	6	24	24
Total operating revenues	934	924	2,752	2,648
Claims, Benefits and Expenses
Net incurred claims and benefits	496	610	1,434	1,459
Policyholders’ dividends	5	4	15	11
Amortization of deferred acquisition costs	127	122	375	359
Non-insurance warranty expense	0	0	0	0
Other insurance related expenses	133	136	386	393
Other expenses	10	6	31	31
Total claims, benefits and expenses	771	878	2,241	2,253
Core income (loss) before income tax	163	46	511	395
Income tax (expense) benefit on core income (loss)	(36)	(14)	(108)	(132)
Core income (loss)	127	32	403	263
Operating Segments | International
Operating revenues
Net earned premiums	255	226	739	629
Net investment income	14	13	43	38
Non-insurance warranty revenue	0	0	0	0
Other revenues	1	1	0	0
Total operating revenues	270	240	782	667
Claims, Benefits and Expenses
Net incurred claims and benefits	172	200	480	444
Policyholders’ dividends	0	0	0	0
Amortization of deferred acquisition costs	57	36	170	127
Non-insurance warranty expense	0	0	0	0
Other insurance related expenses	36	48	102	106
Other expenses	3	(4)	6	(11)
Total claims, benefits and expenses	268	280	758	666
Core income (loss) before income tax	2	(40)	24	1
Income tax (expense) benefit on core income (loss)	(1)	2	(7)	(9)
Core income (loss)	1	(38)	17	(8)
Operating Segments | Life & Group revenues
Operating revenues
Net earned premiums	133	136	398	404
Net investment income	200	195	598	587
Non-insurance warranty revenue	0	0	0	0
Other revenues	(1)	0	0	1
Total operating revenues	332	331	996	992
Claims, Benefits and Expenses
Net incurred claims and benefits	277	322	907	980
Policyholders’ dividends	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Non-insurance warranty expense	0	0	0	0
Other insurance related expenses	31	32	91	96
Other expenses	2	2	5	5
Total claims, benefits and expenses	310	356	1,003	1,081
Core income (loss) before income tax	22	(25)	(7)	(89)
Income tax (expense) benefit on core income (loss)	10	35	43	108
Core income (loss)	32	10	36	19
Operating Segments | Corporate & Other
Operating revenues
Net earned premiums	0	0	0	0
Net investment income	5	6	16	15
Non-insurance warranty revenue	0	0	0	0
Other revenues	2	0	2	1
Total operating revenues	7	6	18	16
Claims, Benefits and Expenses
Net incurred claims and benefits	(12)	(15)	15	4
Policyholders’ dividends	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Non-insurance warranty expense	0	0	0	0
Other insurance related expenses	(1)	0	(1)	(1)
Other expenses	44	52	153	146
Total claims, benefits and expenses	31	37	167	149
Core income (loss) before income tax	(24)	(31)	(149)	(133)
Income tax (expense) benefit on core income (loss)	4	13	30	51
Core income (loss)	(20)	(18)	(119)	(82)
Eliminations
Operating revenues
Net earned premiums	(1)	0	(1)	(1)
Net investment income	0	0	0	0
Non-insurance warranty revenue	0	0	0	0
Other revenues	0	0	(1)	0
Total operating revenues	(1)	0	(2)	(1)
Claims, Benefits and Expenses
Net incurred claims and benefits	0	0	0	0
Policyholders’ dividends	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Non-insurance warranty expense	0	0	0	0
Other insurance related expenses	(1)	0	(1)	(1)
Other expenses	0	0	(1)	0
Total claims, benefits and expenses	(1)	0	(2)	(1)
Core income (loss) before income tax	0	0	0	0
Income tax (expense) benefit on core income (loss)	0	0	0	0
Core income (loss)	$ 0	$ 0	$ 0	$ 0