Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Net premiums written	$ 5,995	$ 4,076
Net premiums earned	6,597	3,927
Losses and loss expenses	3,674	2,122
Policy benefits	126	142
Policy acquisition costs	1,413	707
Administrative expenses	772	554
Underwriting income (loss)	612	402
Net investment income	674	551
Segment Income (loss)	1,286	953
Net realized gains (losses) including OTTI	(394)	(89)
Interest expense	146	68
(Gains) losses from fair value changes in separate account assets	3	(11)
Other	25	(24)
Other (income) expense	28	(35)
Amortization of purchased intangibles	7	30
Chubb integration expenses	148	0
Income tax expense	124	120
Net income (loss)	439	681
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,302	1,297
Net premiums earned	2,896	1,380
Losses and loss expenses	1,747	915
Policy acquisition costs	482	130
Administrative expenses	266	151
Underwriting income (loss)	401	184
Net investment income	426	258
Segment Income (loss)	827	442
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	871	133
Net premiums earned	1,024	146
Losses and loss expenses	661	111
Policy acquisition costs	249	31
Administrative expenses	88	19
Underwriting income (loss)	26	(15)
Net investment income	47	5
Segment Income (loss)	73	$ (10)
Net realized gains (losses) including OTTI
Interest expense
Other
Amortization of purchased intangibles
Income tax expense
Net income (loss)
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	64	$ 88
Net premiums earned	23	64
Losses and loss expenses	(30)	22
Policy acquisition costs	4	(4)
Administrative expenses	(4)	(1)
Underwriting income (loss)	53	47
Net investment income	5	6
Segment Income (loss)	58	$ 53
Net realized gains (losses) including OTTI
Interest expense
Other
Amortization of purchased intangibles
Income tax expense
Net income (loss)
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,041	$ 1,794
Net premiums earned	1,955	1,637
Losses and loss expenses	1,021	814
Policy acquisition costs	503	389
Administrative expenses	263	256
Underwriting income (loss)	168	178
Net investment income	146	138
Segment Income (loss)	314	$ 316
Net realized gains (losses) including OTTI
Interest expense
Other
Amortization of purchased intangibles
Income tax expense
Net income (loss)
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	201	$ 273
Net premiums earned	202	226
Losses and loss expenses	89	99
Policy acquisition costs	53	54
Administrative expenses	14	12
Underwriting income (loss)	46	61
Net investment income	67	75
Segment Income (loss)	113	$ 136
Net realized gains (losses) including OTTI
Interest expense
Other
Amortization of purchased intangibles
Income tax expense
Net income (loss)
Life Insurance [Member]
Segment Reporting Information [Line Items]
Life underwriting income	64
Net premiums written	516	$ 491
Net premiums earned	497	474
Losses and loss expenses	177	152
Policy benefits	126	142
Policy acquisition costs	122	107
Administrative expenses	72	73
Underwriting income (loss)	0	0
Net investment income	67	66
Segment Income (loss)	67	$ 66
Net realized gains (losses) including OTTI
Interest expense
(Gains) losses from fair value changes in separate account assets	3
Other
Amortization of purchased intangibles
Income tax expense
Net income (loss)
Corporate And Other [Member]
Segment Reporting Information [Line Items]
Losses and loss expenses	9	$ 9
Policy acquisition costs		0
Administrative expenses	73	44
Underwriting income (loss)	(82)	(53)
Net investment income	(84)	3
Segment Income (loss)	(166)	(50)
Net realized gains (losses) including OTTI	(394)	(89)
Interest expense	146	68
(Gains) losses from fair value changes in separate account assets	3	(11)
Other	25	(24)
Amortization of purchased intangibles	7	30
Chubb integration expenses	148
Income tax expense	124	120
Net income (loss)	$ (1,013)	$ (322)