Business Segments (Income Statement Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating revenues
Net earned premiums	$ 1,815	$ 1,734	$ 3,600	$ 3,379
Net investment income	506	475	996	1,020
Non-insurance warranty revenue	248	98	486	191
Other revenues	6	9	16	20
Total operating revenues	2,575	2,316	5,098	4,610
Claims, Benefits and Expenses
Net incurred claims and benefits	1,321	1,275	2,654	2,564
Policyholders’ dividends	6	5	12	9
Amortization of deferred acquisition costs	359	312	655	617
Non-insurance warranty expense	225	72	441	142
Other insurance related expenses	236	261	513	515
Other expenses	97	71	158	136
Total claims, benefits and expenses	2,244	1,996	4,433	3,983
Core income (loss) before income tax	331	320	665	627
Income tax (expense) benefit on core income (loss)	(61)	(81)	(114)	(153)
Core income (loss)	270	239	551	474
Net realized investment gains (losses)	(1)	50	11	86
Income tax (expense) benefit on net realized investment gains (losses)	1	(17)	(1)	(28)
Net realized investment gains (losses), after tax	0	33	10	58
Net income	270	272	561	532
Operating Segments | Specialty
Operating revenues
Net earned premiums	683	678	1,355	1,332
Net investment income	130	117	252	265
Non-insurance warranty revenue	248	98	486	191
Other revenues	0	0	1	1
Total operating revenues	1,061	893	2,094	1,789
Claims, Benefits and Expenses
Net incurred claims and benefits	372	394	751	794
Policyholders’ dividends	1	1	2	2
Amortization of deferred acquisition costs	149	147	294	289
Non-insurance warranty expense	225	72	441	142
Other insurance related expenses	70	71	134	138
Other expenses	12	11	23	21
Total claims, benefits and expenses	829	696	1,645	1,386
Core income (loss) before income tax	232	197	449	403
Income tax (expense) benefit on core income (loss)	(49)	(66)	(95)	(135)
Core income (loss)	183	131	354	268
Operating Segments | Commercial
Operating revenues
Net earned premiums	753	716	1,496	1,377
Net investment income	157	146	306	329
Non-insurance warranty revenue	0	0	0	0
Other revenues	8	9	16	18
Total operating revenues	918	871	1,818	1,724
Claims, Benefits and Expenses
Net incurred claims and benefits	470	426	938	849
Policyholders’ dividends	5	4	10	7
Amortization of deferred acquisition costs	127	120	248	237
Non-insurance warranty expense	0	0	0	0
Other insurance related expenses	126	129	253	257
Other expenses	10	10	21	25
Total claims, benefits and expenses	738	689	1,470	1,375
Core income (loss) before income tax	180	182	348	349
Income tax (expense) benefit on core income (loss)	(37)	(62)	(72)	(118)
Core income (loss)	143	120	276	231
Operating Segments | International
Operating revenues
Net earned premiums	248	206	484	403
Net investment income	15	13	29	25
Non-insurance warranty revenue	0	0	0	0
Other revenues	(1)	(1)	(1)	(1)
Total operating revenues	262	218	512	427
Claims, Benefits and Expenses
Net incurred claims and benefits	166	129	308	244
Policyholders’ dividends	0	0	0	0
Amortization of deferred acquisition costs	83	45	113	91
Non-insurance warranty expense	0	0	0	0
Other insurance related expenses	10	31	66	58
Other expenses	7	(1)	3	(7)
Total claims, benefits and expenses	266	204	490	386
Core income (loss) before income tax	(4)	14	22	41
Income tax (expense) benefit on core income (loss)	(3)	(4)	(6)	(11)
Core income (loss)	(7)	10	16	30
Operating Segments | Life & Group revenues
Operating revenues
Net earned premiums	131	135	265	268
Net investment income	198	195	398	392
Non-insurance warranty revenue	0	0	0	0
Other revenues	0	0	1	1
Total operating revenues	329	330	664	661
Claims, Benefits and Expenses
Net incurred claims and benefits	327	328	630	658
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	30	32	60	64
Other expenses	1	1	3	3
Total claims, benefits and expenses	358	361	693	725
Core income (loss) before income tax	(29)	(31)	(29)	(64)
Income tax (expense) benefit on core income (loss)	19	36	33	73
Core income (loss)	(10)	5	4	9
Operating Segments | Corporate & Other
Operating revenues
Net earned premiums	0	0	0	0
Net investment income	6	4	11	9
Non-insurance warranty revenue	0	0	0	0
Other revenues	(1)	1	0	1
Total operating revenues	5	5	11	10
Claims, Benefits and Expenses
Net incurred claims and benefits	(14)	(2)	27	19
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	0	(1)	0	(1)
Other expenses	67	50	109	94
Total claims, benefits and expenses	53	47	136	112
Core income (loss) before income tax	(48)	(42)	(125)	(102)
Income tax (expense) benefit on core income (loss)	9	15	26	38
Core income (loss)	(39)	(27)	(99)	(64)
Eliminations
Operating revenues
Net earned premiums	0	(1)	0	(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	0	(1)	(1)	(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	0	(1)	0	(1)
Other expenses	0	0	(1)	0
Total claims, benefits and expenses	0	(1)	(1)	(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