Business Segments (Income Statement Information) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net written premiums		$ 7,069	$ 6,988	$ 6,962
Operating revenues
Net earned premiums		6,988	6,924	6,921
Net investment income		2,034	1,988	1,840
Other revenues		427	404	407
Total operating revenues		9,449	9,316	9,168
Claims, Benefits and Expenses
Net incurred claims and benefits		5,288	5,270	5,372
Policyholders’ dividends		22	13	12
Amortization of deferred acquisition costs		1,233	1,235	1,540
Other insurance related expenses		1,098	1,122	1,093
Other expenses		591	589	535
Total claims, benefits and expenses		8,232	8,229	8,552
Core income (loss) before income tax		1,217	1,087	616
Income tax (expense) benefit on core income (loss)		(298)	(263)	(101)
Core income (loss)		919	824	515
Net realized investment gains (losses)		93	50	(67)
Income tax (expense) benefit on net realized investment gains (losses)		(30)	(15)	31
Net realized investment gains (losses), after tax		63	35	(36)
Net deferred tax asset remeasurement	$ (83)	(83)	0	0
Net income		899	859	479
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Net written premiums		2,771	2,780	2,781
Operating revenues
Net earned premiums		2,753	2,779	2,782
Net investment income		538	516	474
Other revenues		392	362	356
Total operating revenues		3,683	3,657	3,612
Claims, Benefits and Expenses
Net incurred claims and benefits		1,535	1,467	1,597
Policyholders’ dividends		4	4	4
Amortization of deferred acquisition costs		596	591	589
Other insurance related expenses		286	301	278
Other expenses		342	312	301
Total claims, benefits and expenses		2,763	2,675	2,769
Core income (loss) before income tax		920	982	843
Income tax (expense) benefit on core income (loss)		(310)	(332)	(283)
Core income (loss)		610	650	560
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Net written premiums		2,882	2,841	2,818
Operating revenues
Net earned premiums		2,840	2,804	2,788
Net investment income		642	638	593
Other revenues		31	32	37
Total operating revenues		3,513	3,474	3,418
Claims, Benefits and Expenses
Net incurred claims and benefits		1,928	1,927	1,814
Policyholders’ dividends		18	9	8
Amortization of deferred acquisition costs		475	470	469
Other insurance related expenses		523	560	538
Other expenses		57	36	28
Total claims, benefits and expenses		3,001	3,002	2,857
Core income (loss) before income tax		512	472	561
Income tax (expense) benefit on core income (loss)		(171)	(161)	(192)
Core income (loss)		341	311	369
Operating Segments | International
Segment Reporting Information [Line Items]
Net written premiums		881	821	822
Operating revenues
Net earned premiums		857	806	804
Net investment income		52	51	52
Other revenues		0	0	(1)
Total operating revenues		909	857	855
Claims, Benefits and Expenses
Net incurred claims and benefits		575	492	479
Policyholders’ dividends		0	0	0
Amortization of deferred acquisition costs		162	174	168
Other insurance related expenses		162	133	138
Other expenses		(7)	24	12
Total claims, benefits and expenses		892	823	797
Core income (loss) before income tax		17	34	58
Income tax (expense) benefit on core income (loss)		(9)	(13)	(21)
Core income (loss)		8	21	37
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Net written premiums		536	547	542
Operating revenues
Net earned premiums		539	536	548
Net investment income		782	767	704
Other revenues		2	(2)	7
Total operating revenues		1,323	1,301	1,259
Claims, Benefits and Expenses
Net incurred claims and benefits		1,269	1,286	1,421
Policyholders’ dividends		0	0	0
Amortization of deferred acquisition costs		0	0	314
Other insurance related expenses		129	132	142
Other expenses		7	8	11
Total claims, benefits and expenses		1,405	1,426	1,888
Core income (loss) before income tax		(82)	(125)	(629)
Income tax (expense) benefit on core income (loss)		132	145	315
Core income (loss)		50	20	(314)
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Net written premiums		0	0	1
Operating revenues
Net earned premiums		0	0	1
Net investment income		20	16	17
Other revenues		2	12	11
Total operating revenues		22	28	29
Claims, Benefits and Expenses
Net incurred claims and benefits		(19)	98	61
Policyholders’ dividends		0	0	0
Amortization of deferred acquisition costs		0	0	0
Other insurance related expenses		(1)	(3)	(1)
Other expenses		192	209	186
Total claims, benefits and expenses		172	304	246
Core income (loss) before income tax		(150)	(276)	(217)
Income tax (expense) benefit on core income (loss)		60	98	80
Core income (loss)		(90)	(178)	(137)
Eliminations
Segment Reporting Information [Line Items]
Net written premiums		(1)	(1)	(2)
Operating revenues
Net earned premiums		(1)	(1)	(2)
Net investment income		0	0	0
Other revenues		0	0	(3)
Total operating revenues		(1)	(1)	(5)
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
Other insurance related expenses		(1)	(1)	(2)
Other expenses		0	0	(3)
Total claims, benefits and expenses		(1)	(1)	(5)
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