Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Net premiums written	$ 7,581	$ 7,639	$ 14,291	$ 13,634
Net premiums earned	7,237	7,405	14,009	14,002
Losses and loss expenses	4,146	4,254	7,935	7,928
Policy benefits	163	146	331	272
Policy acquisition costs	1,449	1,560	2,846	2,973
Administrative expenses	706	829	1,382	1,601
Underwriting income (loss)	773	616	1,515	1,228
Net investment income	770	708	1,515	1,382
Other (income) expense	(145)	(29)	(215)	(1)
Amortization of purchased intangibles	65	5	129	12
Segment Income (loss)	1,623	1,348	3,116	2,599
Net realized gains (losses) including OTTI	101	(216)	94	(610)
Interest expense	147	153	301	299
Chubb integration expenses	72	98	183	246
Income tax expense	200	155	328	279
Net income	1,305	726	2,398	1,165
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,204	3,245	5,946	5,547
Net premiums earned	3,099	3,148	6,140	6,044
Losses and loss expenses	1,936	1,971	3,796	3,718
Policy benefits	0		0	0
Policy acquisition costs	464	545	951	1,027
Administrative expenses	241	299	472	565
Underwriting income (loss)	458	333	921	734
Net investment income	490	468	968	894
Other (income) expense	(4)	(9)	0	(9)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	952	810	1,889	1,637
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,255	1,231	2,239	2,102
Net premiums earned	1,093	1,140	2,179	2,164
Losses and loss expenses	683	661	1,316	1,322
Policy benefits	0		0	0
Policy acquisition costs	230	269	447	518
Administrative expenses	66	98	131	186
Underwriting income (loss)	114	112	285	138
Net investment income	56	55	111	102
Other (income) expense	1	3	2	4
Amortization of purchased intangibles	5	4	8	12
Segment Income (loss)	164	160	386	224
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	403	375	464	439
Net premiums earned	344	327	358	350
Losses and loss expenses	290	284	217	254
Policy benefits			0	0
Policy acquisition costs	27	25	26	29
Administrative expenses	2	2	(3)	(2)
Underwriting income (loss)	25	16	118	69
Net investment income	6	5	12	10
Other (income) expense	1	0	1	0
Amortization of purchased intangibles	7	8	14	15
Segment Income (loss)	23	13	115	64
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,006	2,031	4,206	4,072
Net premiums earned	2,018	2,093	3,954	4,048
Losses and loss expenses	964	1,089	2,035	2,110
Policy benefits	0		0	0
Policy acquisition costs	555	537	1,084	1,040
Administrative expenses	243	277	488	540
Underwriting income (loss)	256	190	347	358
Net investment income	148	147	296	293
Other (income) expense	(3)	(5)	(4)	(10)
Amortization of purchased intangibles	11	13	22	24
Segment Income (loss)	396	329	625	637
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	190	230	389	431
Net premiums earned	168	185	357	387
Losses and loss expenses	46	87	140	176
Policy benefits	0		0	0
Policy acquisition costs	43	47	94	100
Administrative expenses	12	14	22	28
Underwriting income (loss)	67	37	101	83
Net investment income	65	65	127	132
Other (income) expense	1	(2)	1	(3)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	131	104	227	218
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	523	527	1,047	1,043
Net premiums earned	515	512	1,021	1,009
Losses and loss expenses	182	147	375	324
Policy benefits	163	146	331	272
Policy acquisition costs	130	137	244	259
Administrative expenses	77	77	149	149
Underwriting income (loss)	(37)	5	(78)	5
Net investment income	77	69	152	136
Other (income) expense	(12)	0	(41)	6
Amortization of purchased intangibles	0	0	1	1
Segment Income (loss)	52	74	114	134
Corporate And Other [Member]
Segment Reporting Information [Line Items]
Net premiums written	0		0	0
Net premiums earned	0		0	0
Losses and loss expenses	45	15	56	24
Policy benefits	0		0	0
Policy acquisition costs	0	0	0	0
Administrative expenses	65	62	123	135
Underwriting income (loss)	(110)	(77)	(179)	(159)
Net investment income	(72)	(101)	(151)	(185)
Other (income) expense	(129)	(16)	(174)	11
Amortization of purchased intangibles	42	(20)	84	(40)
Segment Income (loss)	(95)	(142)	(240)	(315)
Net realized gains (losses) including OTTI	101	(216)	94	(610)
Interest expense	147	153	301	299
Chubb integration expenses	72	98	183	246
Income tax expense	200	155	328	279
Net income	$ (413)	$ (764)	$ (958)	$ (1,749)