Business Segments - Income Statement Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating revenues
Net earned premiums	$ 1,806	$ 1,767	$ 5,185	$ 5,196
Net investment income	509	524	1,529	1,461
Other revenues	107	96	318	293
Total operating revenues	2,422	2,387	7,032	6,950
Claims, Benefits and Expenses
Net incurred claims and benefits	1,475	1,195	4,039	3,934
Policyholders’ dividends	5	7	14	15
Amortization of deferred acquisition costs	309	314	926	926
Other insurance related expenses	281	295	796	842
Other expenses	141	147	419	439
Total claims, benefits and expenses	2,211	1,958	6,194	6,156
Operating income (loss) before income tax	211	429	838	794
Income tax (expense) benefit on operating income (loss)	(52)	(118)	(205)	(191)
Net operating income (loss)	159	311	633	603
Net realized investment gains (losses)	(24)	46	62	26
Income tax (expense) benefit on net realized investment gains (losses)	9	(14)	(19)	(11)
Net realized investment gains (losses), after tax	(15)	32	43	15
Net income (loss)	144	343	676	618
Operating Segments | Specialty
Operating revenues
Net earned premiums	703	704	2,056	2,088
Net investment income	134	140	407	380
Other revenues	99	93	291	269
Total operating revenues	936	937	2,754	2,737
Claims, Benefits and Expenses
Net incurred claims and benefits	357	330	1,141	1,097
Policyholders’ dividends	1	4	3	6
Amortization of deferred acquisition costs	153	151	445	443
Other insurance related expenses	68	77	209	225
Other expenses	85	78	248	232
Total claims, benefits and expenses	664	640	2,046	2,003
Operating income (loss) before income tax	272	297	708	734
Income tax (expense) benefit on operating income (loss)	(92)	(102)	(238)	(248)
Net operating income (loss)	180	195	470	486
Net realized investment gains (losses)	4	9	25	2
Income tax (expense) benefit on net realized investment gains (losses)	(1)	(3)	(9)	(1)
Net realized investment gains (losses), after tax	3	6	16	1
Net income (loss)	183	201	486	487
Operating Segments | Commercial
Operating revenues
Net earned premiums	741	719	2,097	2,103
Net investment income	161	175	482	465
Other revenues	7	7	25	21
Total operating revenues	909	901	2,604	2,589
Claims, Benefits and Expenses
Net incurred claims and benefits	611	446	1,470	1,357
Policyholders’ dividends	4	3	11	9
Amortization of deferred acquisition costs	120	118	354	351
Other insurance related expenses	133	151	387	422
Other expenses	6	9	31	25
Total claims, benefits and expenses	874	727	2,253	2,164
Operating income (loss) before income tax	35	174	351	425
Income tax (expense) benefit on operating income (loss)	(10)	(60)	(117)	(145)
Net operating income (loss)	25	114	234	280
Net realized investment gains (losses)	6	12	36	2
Income tax (expense) benefit on net realized investment gains (losses)	(3)	(3)	(12)	0
Net realized investment gains (losses), after tax	3	9	24	2
Net income (loss)	28	123	258	282
Operating Segments | International
Operating revenues
Net earned premiums	226	210	629	605
Net investment income	13	13	38	38
Other revenues	1	0	0	1
Total operating revenues	240	223	667	644
Claims, Benefits and Expenses
Net incurred claims and benefits	200	117	444	395
Policyholders’ dividends	0	0	0	0
Amortization of deferred acquisition costs	36	45	127	132
Other insurance related expenses	48	33	106	98
Other expenses	(4)	1	(11)	17
Total claims, benefits and expenses	280	196	666	642
Operating income (loss) before income tax	(40)	27	1	2
Income tax (expense) benefit on operating income (loss)	2	(7)	(9)	(3)
Net operating income (loss)	(38)	20	(8)	(1)
Net realized investment gains (losses)	4	6	17	14
Income tax (expense) benefit on net realized investment gains (losses)	(1)	(1)	(3)	(3)
Net realized investment gains (losses), after tax	3	5	14	11
Net income (loss)	(35)	25	6	10
Operating Segments | Life & Group Non-Core
Operating revenues
Net earned premiums	136	134	404	401
Net investment income	195	192	587	567
Other revenues	0	(4)	1	(1)
Total operating revenues	331	322	992	967
Claims, Benefits and Expenses
Net incurred claims and benefits	322	313	980	976
Policyholders’ dividends	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Other insurance related expenses	32	37	96	101
Other expenses	2	2	5	7
Total claims, benefits and expenses	356	352	1,081	1,084
Operating income (loss) before income tax	(25)	(30)	(89)	(117)
Income tax (expense) benefit on operating income (loss)	35	36	108	117
Net operating income (loss)	10	6	19	0
Net realized investment gains (losses)	3	17	20	12
Income tax (expense) benefit on net realized investment gains (losses)	(1)	(6)	(8)	(9)
Net realized investment gains (losses), after tax	2	11	12	3
Net income (loss)	12	17	31	3
Operating Segments | Corporate & Other Non-Core
Operating revenues
Net earned premiums	0	0	0	0
Net investment income	6	4	15	11
Other revenues	0	0	1	3
Total operating revenues	6	4	16	14
Claims, Benefits and Expenses
Net incurred claims and benefits	(15)	(11)	4	109
Policyholders’ dividends	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Other insurance related expenses	0	(3)	(1)	(3)
Other expenses	52	57	146	158
Total claims, benefits and expenses	37	43	149	264
Operating income (loss) before income tax	(31)	(39)	(133)	(250)
Income tax (expense) benefit on operating income (loss)	13	15	51	88
Net operating income (loss)	(18)	(24)	(82)	(162)
Net realized investment gains (losses)	(41)	2	(36)	(4)
Income tax (expense) benefit on net realized investment gains (losses)	15	(1)	13	2
Net realized investment gains (losses), after tax	(26)	1	(23)	(2)
Net income (loss)	(44)	(23)	(105)	(164)
Eliminations
Operating revenues
Net earned premiums	0	0	(1)	(1)
Net investment income	0	0	0	0
Other revenues	0	0	0	0
Total operating revenues	0	0	(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
Other insurance related expenses	0	0	(1)	(1)
Other expenses	0	0	0	0
Total claims, benefits and expenses	0	0	(1)	(1)
Operating income (loss) before income tax	0	0	0	0
Income tax (expense) benefit on operating income (loss)	0	0	0	0
Net operating income (loss)	0	0	0	0
Net realized investment gains (losses)	0	0	0	0
Income tax (expense) benefit on net realized investment gains (losses)	0	0	0	0
Net realized investment gains (losses), after tax	0	0	0	0
Net income (loss)	$ 0	$ 0	$ 0	$ 0