Business Segments - Income Statement Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Operating revenues
Net earned premiums	$ 1,734	$ 1,730	$ 3,379	$ 3,429
Net investment income	475	502	1,020	937
Other revenues	107	100	211	197
Total operating revenues	2,316	2,332	4,610	4,563
Claims, Benefits and Expenses
Net incurred claims and benefits	1,275	1,335	2,564	2,739
Policyholders’ dividends	5	4	9	8
Amortization of deferred acquisition costs	312	305	617	612
Other insurance related expenses	261	270	515	547
Other expenses	143	146	278	292
Total claims, benefits and expenses	1,996	2,060	3,983	4,198
Operating income (loss) before income tax	320	272	627	365
Income tax (expense) benefit on operating income (loss)	(81)	(71)	(153)	(73)
Net operating income (loss)	239	201	474	292
Net realized investment gains (losses)	50	16	86	(20)
Income tax (expense) benefit on net realized investment gains (losses)	(17)	(8)	(28)	3
Net realized investment gains (losses), after tax	33	8	58	(17)
Net income (loss)	272	209	532	275
Operating Segments | Specialty
Operating revenues
Net earned premiums	689	702	1,353	1,384
Net investment income	120	133	273	240
Other revenues	98	89	192	176
Total operating revenues	907	924	1,818	1,800
Claims, Benefits and Expenses
Net incurred claims and benefits	398	377	784	767
Policyholders’ dividends	1	1	2	2
Amortization of deferred acquisition costs	149	148	292	292
Other insurance related expenses	72	73	141	148
Other expenses	82	79	163	154
Total claims, benefits and expenses	702	678	1,382	1,363
Operating income (loss) before income tax	205	246	436	437
Income tax (expense) benefit on operating income (loss)	(69)	(82)	(146)	(146)
Net operating income (loss)	136	164	290	291
Net realized investment gains (losses)	14	4	21	(7)
Income tax (expense) benefit on net realized investment gains (losses)	(5)	(2)	(8)	2
Net realized investment gains (losses), after tax	9	2	13	(5)
Net income (loss)	145	166	303	286
Operating Segments | Commercial
Operating revenues
Net earned premiums	705	696	1,356	1,384
Net investment income	143	164	321	290
Other revenues	9	8	18	14
Total operating revenues	857	868	1,695	1,688
Claims, Benefits and Expenses
Net incurred claims and benefits	422	469	859	911
Policyholders’ dividends	4	3	7	6
Amortization of deferred acquisition costs	118	117	234	233
Other insurance related expenses	128	130	254	271
Other expenses	11	11	25	16
Total claims, benefits and expenses	683	730	1,379	1,437
Operating income (loss) before income tax	174	138	316	251
Income tax (expense) benefit on operating income (loss)	(59)	(46)	(107)	(85)
Net operating income (loss)	115	92	209	166
Net realized investment gains (losses)	19	8	30	(10)
Income tax (expense) benefit on net realized investment gains (losses)	(6)	(3)	(9)	3
Net realized investment gains (losses), after tax	13	5	21	(7)
Net income (loss)	128	97	230	159
Operating Segments | International
Operating revenues
Net earned premiums	206	197	403	395
Net investment income	13	13	25	25
Other revenues	(1)	0	(1)	1
Total operating revenues	218	210	427	421
Claims, Benefits and Expenses
Net incurred claims and benefits	129	157	244	278
Policyholders’ dividends	0	0	0	0
Amortization of deferred acquisition costs	45	40	91	87
Other insurance related expenses	31	37	58	65
Other expenses	(1)	7	(7)	16
Total claims, benefits and expenses	204	241	386	446
Operating income (loss) before income tax	14	(31)	41	(25)
Income tax (expense) benefit on operating income (loss)	(4)	4	(11)	4
Net operating income (loss)	10	(27)	30	(21)
Net realized investment gains (losses)	7	4	13	8
Income tax (expense) benefit on net realized investment gains (losses)	(1)	(1)	(2)	(2)
Net realized investment gains (losses), after tax	6	3	11	6
Net income (loss)	16	(24)	41	(15)
Operating Segments | Life & Group Non-Core
Operating revenues
Net earned premiums	135	136	268	267
Net investment income	195	188	392	375
Other revenues	0	3	1	3
Total operating revenues	330	327	661	645
Claims, Benefits and Expenses
Net incurred claims and benefits	328	340	658	663
Policyholders’ dividends	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Other insurance related expenses	32	31	64	64
Other expenses	1	2	3	5
Total claims, benefits and expenses	361	373	725	732
Operating income (loss) before income tax	(31)	(46)	(64)	(87)
Income tax (expense) benefit on operating income (loss)	36	42	73	81
Net operating income (loss)	5	(4)	9	(6)
Net realized investment gains (losses)	7	(2)	17	(5)
Income tax (expense) benefit on net realized investment gains (losses)	(3)	(3)	(7)	(3)
Net realized investment gains (losses), after tax	4	(5)	10	(8)
Net income (loss)	9	(9)	19	(14)
Operating Segments | Corporate & Other Non-Core
Operating revenues
Net earned premiums	0	0	0	0
Net investment income	4	4	9	7
Other revenues	1	0	1	3
Total operating revenues	5	4	10	10
Claims, Benefits and Expenses
Net incurred claims and benefits	(2)	(8)	19	120
Policyholders’ dividends	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Other insurance related expenses	(1)	0	(1)	0
Other expenses	50	47	94	101
Total claims, benefits and expenses	47	39	112	221
Operating income (loss) before income tax	(42)	(35)	(102)	(211)
Income tax (expense) benefit on operating income (loss)	15	11	38	73
Net operating income (loss)	(27)	(24)	(64)	(138)
Net realized investment gains (losses)	3	2	5	(6)
Income tax (expense) benefit on net realized investment gains (losses)	(2)	1	(2)	3
Net realized investment gains (losses), after tax	1	3	3	(3)
Net income (loss)	(26)	(21)	(61)	(141)
Intersegment Eliminations
Operating revenues
Net earned premiums	(1)	(1)	(1)	(1)
Net investment income	0	0	0	0
Other revenues	0	0	0	0
Total operating revenues	(1)	(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
Other insurance related expenses	(1)	(1)	(1)	(1)
Other expenses	0	0	0	0
Total claims, benefits and expenses	(1)	(1)	(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