Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Net premiums written	$ 7,573	$ 4,709	$ 21,207	$ 13,569
Net premiums earned	7,688	4,719	21,690	13,006
Losses and loss expenses	4,269	2,643	12,197	7,182
Policy benefits	155	89	427	384
Policy acquisition costs	1,514	771	4,487	2,205
Administrative expenses	772	568	2,373	1,700
Underwriting income (loss)	978	648	2,206	1,535
Net investment income	739	549	2,121	1,662
Other (income) expense	(91)	12	(92)	(61)
Amortization of purchased intangibles	4	51	16	136
Segment Income (loss)	1,804	1,134	4,403	3,122
Net realized gains including OTTI	100	(397)	(510)	(360)
Interest expense	152	68	451	207
Chubb integration expenses	115	9	361	9
Income tax expense	277	132	556	395
Net income	1,360	528	2,525	2,151
Gain (Loss) on Derivative	48	(344)	(442)	(272)
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,110	1,433	8,657	4,158
Net premiums earned	3,086	1,410	9,130	4,209
Losses and loss expenses	1,863	913	5,581	2,744
Policy acquisition costs	522	142	1,549	403
Administrative expenses	275	154	840	459
Underwriting income (loss)	426	201	1,160	603
Net investment income	477	260	1,371	780
Other (income) expense	3	(3)	(6)	(5)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	900	464	2,537	1,388
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,011	278	3,113	958
Net premiums earned	1,081	272	3,245	687
Losses and loss expenses	594	179	1,916	446
Policy acquisition costs	229	13	747	43
Administrative expenses	89	36	275	89
Underwriting income (loss)	169	44	307	109
Net investment income	53	7	155	19
Other (income) expense	2	1	6	1
Amortization of purchased intangibles	4	31	16	63
Segment Income (loss)	216	19	440	64
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	849	737	1,288	1,204
Net premiums earned	819	739	1,169	1,124
Losses and loss expenses	683	620	937	913
Policy acquisition costs	48	42	77	61
Administrative expenses	1	0	(1)	3
Underwriting income (loss)	87	77	156	147
Net investment income	5	5	15	17
Other (income) expense	0	(1)	0	1
Amortization of purchased intangibles	7	8	22	22
Segment Income (loss)	85	75	149	141
Management North America Agricultural Insurance segment income	90
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,940	1,584	6,012	5,047
Net premiums earned	2,034	1,615	6,082	4,896
Losses and loss expenses	843	674	2,953	2,304
Policy acquisition costs	546	405	1,586	1,190
Administrative expenses	261	246	801	756
Underwriting income (loss)	384	290	742	646
Net investment income	152	132	445	409
Other (income) expense	(6)	(6)	(16)	(12)
Amortization of purchased intangibles	12	12	36	50
Segment Income (loss)	530	416	1,167	1,017
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	131	185	562	719
Net premiums earned	156	203	543	649
Losses and loss expenses	49	20	225	191
Policy acquisition costs	42	52	142	166
Administrative expenses	12	12	40	37
Underwriting income (loss)	53	119	136	255
Net investment income	67	76	199	230
Other (income) expense	0	(4)	(3)	(5)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	120	199	338	490
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	532	492	1,575	1,483
Net premiums earned	512	480	1,521	1,441
Losses and loss expenses	174	153	498	442
Policy benefits	155	89	427	384
Policy acquisition costs	127	117	386	342
Administrative expenses	77	74	226	221
Underwriting income (loss)	(21)	47	(16)	52
Net investment income	71	66	207	198
Other (income) expense	(20)	48	(14)	32
Amortization of purchased intangibles	1	0	2	1
Segment Income (loss)	69	65	203	217
Management Underwriting Income (Loss)	72
Gains Losses On Fair Value Changes In Separate Account Assets	22
Corporate And Other [Member]
Segment Reporting Information [Line Items]
Losses and loss expenses	63	84	87	142
Policy acquisition costs		0		0
Administrative expenses	57	46	192	135
Underwriting income (loss)	(120)	(130)	(279)	(277)
Net investment income	(86)	3	(271)	9
Other (income) expense	(70)	(23)	(59)	(73)
Amortization of purchased intangibles	(20)	0	(60)	0
Segment Income (loss)	(116)	(104)	(431)	(195)
Net realized gains including OTTI	100	(397)	(510)	(360)
Interest expense	152	68	451	207
Chubb integration expenses	115	9	361	9
Income tax expense	277	132	556	395
Net income	(560)	$ (710)	$ (2,309)	$ (1,166)
Commodity [Member] | North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Derivative	$ 3