Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Net premiums written	$ 7,639	$ 4,784	$ 13,634	$ 8,860
Net premiums earned	7,405	4,360	14,002	8,287
Losses and loss expenses	4,254	2,417	7,928	4,539
Policy benefits	146	153	272	295
Policy acquisition costs	1,560	727	2,973	1,434
Administrative expenses	829	578	1,601	1,132
Underwriting income (loss)	616	485	1,228	887
Net investment income	708	562	1,382	1,113
Other (income) expense	(29)	(38)	(1)	(73)
Segment Income (loss)	1,353	1,085	2,611	2,073
Net realized gains (losses) including OTTI	(216)	126	(610)	37
Interest expense	153	71	299	139
Amortization of purchased intangibles	5	55	12	85
Chubb integration expenses	98	0	246	0
Income tax expense	155	143	279	263
Net income (loss)	726	942	1,165	1,623
North America Agricultural Insurance segment income	14
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,245	1,428	5,547	2,725
Net premiums earned	3,148	1,419	6,044	2,799
Losses and loss expenses	1,971	916	3,718	1,831
Policy acquisition costs	545	131	1,027	261
Administrative expenses	299	154	565	305
Underwriting income (loss)	333	218	734	402
Net investment income	468	262	894	520
Other (income) expense	(9)	1	(9)	(2)
Segment Income (loss)	810	479	1,637	924
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,231	547	2,102	680
Net premiums earned	1,140	269	2,164	415
Losses and loss expenses	661	156	1,322	267
Policy acquisition costs	269	(1)	518	30
Administrative expenses	98	34	186	53
Underwriting income (loss)	112	80	138	65
Net investment income	55	7	102	12
Other (income) expense	3	0	4	0
Segment Income (loss)	164	87	236	77
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	375	379	439	467
Net premiums earned	327	321	350	385
Losses and loss expenses	284	271	254	293
Policy acquisition costs	25	23	29	19
Administrative expenses	2	4	(2)	3
Underwriting income (loss)	16	23	69	70
Net investment income	5	6	10	12
Other (income) expense	0	1	0	2
Segment Income (loss)	21	28	79	80
Net realized gains (losses) including OTTI	2
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,031	1,669	4,072	3,463
Net premiums earned	2,093	1,644	4,048	3,281
Losses and loss expenses	1,089	816	2,110	1,630
Policy acquisition costs	537	396	1,040	785
Administrative expenses	277	254	540	510
Underwriting income (loss)	190	178	358	356
Net investment income	147	139	293	277
Other (income) expense	(5)	(4)	(10)	(6)
Segment Income (loss)	342	321	661	639
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	230	261	431	534
Net premiums earned	185	220	387	446
Losses and loss expenses	87	72	176	171
Policy acquisition costs	47	60	100	114
Administrative expenses	14	13	28	25
Underwriting income (loss)	37	75	83	136
Net investment income	65	79	132	154
Other (income) expense	(2)	0	(3)	(1)
Segment Income (loss)	104	154	218	291
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	527	500	1,043	991
Net premiums earned	512	487	1,009	961
Losses and loss expenses	147	137	324	289
Policy benefits	146	153	272	295
Policy acquisition costs	137	118	259	225
Administrative expenses	77	74	149	147
Underwriting income (loss)	5	5	5	5
Net investment income	69	66	136	132
Other (income) expense	0	(7)	6	(16)
Segment Income (loss)	74	78	135	153
Corporate And Other [Member]
Segment Reporting Information [Line Items]
Losses and loss expenses	15	49	24	58
Policy acquisition costs		0		0
Administrative expenses	62	45	135	89
Underwriting income (loss)	(77)	(94)	(159)	(147)
Net investment income	(101)	3	(185)	6
Other (income) expense	(16)	(29)	11	(50)
Segment Income (loss)	(162)	(62)	(355)	(91)
Net realized gains (losses) including OTTI	(216)	126	(610)	37
Interest expense	153	71	299	139
Amortization of purchased intangibles	5	55	12	85
Chubb integration expenses	98		246
Income tax expense	155	143	279	263
Net income (loss)	$ (789)	$ (205)	$ (1,801)	$ (541)