Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Net premiums written	$ 7,902	$ 7,573	$ 22,193	$ 21,207
Net premiums earned	7,807	7,688	21,816	21,690
Losses and loss expenses	6,247	4,269	14,182	12,197
Policy benefits	169	155	500	427
Policy acquisition costs	1,488	1,514	4,334	4,487
Administrative expenses	714	772	2,096	2,373
Underwriting income (loss)	(811)	978	704	2,206
Net investment income	813	739	2,328	2,121
Other (income) expense	(118)	(91)	(333)	(92)
Amortization of purchased intangibles	65	4	194	16
Segment Income (loss)	55	1,804	3,171	4,403
Net realized gains (losses) including OTTI	(10)	100	84	(510)
Interest expense	150	152	451	451
Chubb integration expenses	50	115	233	361
Income tax benefit	(85)	277	243	556
Net income (loss)	(70)	1,360	2,328	2,525
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,089	3,110	9,035	8,657
Net premiums earned	3,016	3,086	9,156	9,130
Losses and loss expenses	2,580	1,863	6,376	5,581
Policy benefits	0		0	0
Policy acquisition costs	469	522	1,420	1,549
Administrative expenses	256	275	728	840
Underwriting income (loss)	(289)	426	632	1,160
Net investment income	497	477	1,465	1,371
Other (income) expense	(4)	3	(4)	(6)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	212	900	2,101	2,537
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,194	1,011	3,433	3,113
Net premiums earned	1,117	1,081	3,296	3,245
Losses and loss expenses	1,062	594	2,378	1,916
Policy benefits	0		0	0
Policy acquisition costs	226	229	673	747
Administrative expenses	61	89	192	275
Underwriting income (loss)	(232)	169	53	307
Net investment income	57	53	168	155
Other (income) expense	1	2	3	6
Amortization of purchased intangibles	4	4	12	16
Segment Income (loss)	(180)	216	206	440
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	926	849	1,390	1,288
Net premiums earned	898	819	1,256	1,169
Losses and loss expenses	759	683	976	937
Policy benefits	0		0	0
Policy acquisition costs	49	48	75	77
Administrative expenses	(1)	1	(4)	(1)
Underwriting income (loss)	91	87	209	156
Net investment income	6	5	18	15
Other (income) expense	0	0	1	0
Amortization of purchased intangibles	8	7	22	22
Segment Income (loss)	89	85	204	149
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,963	1,940	6,169	6,012
Net premiums earned	2,064	2,034	6,018	6,082
Losses and loss expenses	1,281	843	3,316	2,953
Policy benefits	0		0	0
Policy acquisition costs	569	546	1,653	1,586
Administrative expenses	246	261	734	801
Underwriting income (loss)	(32)	384	315	742
Net investment income	164	152	460	445
Other (income) expense	(10)	(6)	(14)	(16)
Amortization of purchased intangibles	11	12	33	36
Segment Income (loss)	131	530	756	1,167
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	191	131	580	562
Net premiums earned	185	156	542	543
Losses and loss expenses	295	49	435	225
Policy benefits	0		0	0
Policy acquisition costs	43	42	137	142
Administrative expenses	11	12	33	40
Underwriting income (loss)	(164)	53	(63)	136
Net investment income	80	67	207	199
Other (income) expense	(3)	0	(2)	(3)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	(81)	120	146	338
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	539	532	1,586	1,575
Net premiums earned	527	512	1,548	1,521
Losses and loss expenses	181	174	556	498
Policy benefits	169	155	500	427
Policy acquisition costs	132	127	376	386
Administrative expenses	77	77	226	226
Underwriting income (loss)	(32)	(21)	(110)	(16)
Net investment income	78	71	230	207
Other (income) expense	(19)	(20)	(60)	(14)
Amortization of purchased intangibles	1	1	2	2
Segment Income (loss)	64	69	178	203
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	89	63	145	87
Policy benefits	0		0	0
Policy acquisition costs	0	0	0	0
Administrative expenses	64	57	187	192
Underwriting income (loss)	(153)	(120)	(332)	(279)
Net investment income	(69)	(86)	(220)	(271)
Other (income) expense	(83)	(70)	(257)	(59)
Amortization of purchased intangibles	41	(20)	125	(60)
Segment Income (loss)	(180)	(116)	(420)	(431)
Net realized gains (losses) including OTTI	(10)	100	84	(510)
Interest expense	150	152	451	451
Chubb integration expenses	50	115	233	361
Income tax benefit	(85)	277	243	556
Net income (loss)	$ (305)	$ (560)	$ (1,263)	$ (2,309)