Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Net premiums written	$ 8,662	$ 7,977
Net premiums earned	8,221	7,794
Losses and loss expenses	5,053	4,485
Policy benefits	167	129
Policy acquisition costs	1,665	1,615
Administrative expenses	744	741
Underwriting income (loss)	592	824
Net investment income (loss)	863	861
Other (income) expense	(490)	55
Amortization of purchased intangibles	72	73
Segment Income (loss)	1,873	1,557
Realized Investment Gains (Losses)	887	(958)
Interest expense	122	132
Income tax expense	338	215
Net income	2,300	252
Segment Reconciling Items
Segment Reporting Information [Line Items]
Net premiums written	0	0
Net premiums earned	0	0
Losses and loss expenses	(1)	(2)
Policy benefits	4	(56)
Policy acquisition costs	0	0
Administrative expenses	0	0
Underwriting income (loss)	(3)	58
Net investment income (loss)	(41)	(24)
Other (income) expense	(45)	32
Amortization of purchased intangibles	0	0
Segment Income (loss)	1	2
Realized Investment Gains (Losses)	(1)	(2)
Interest expense	0	0
Income tax expense	0	0
Net income	0	0
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,664	3,252
Net premiums earned	3,674	3,376
Losses and loss expenses	2,560	2,181
Policy benefits	0	0
Policy acquisition costs	514	492
Administrative expenses	254	259
Underwriting income (loss)	346	444
Net investment income (loss)	540	525
Other (income) expense	2	6
Amortization of purchased intangibles	0	0
Segment Income (loss)	884	963
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,098	1,107
Net premiums earned	1,184	1,200
Losses and loss expenses	819	683
Policy benefits	0	0
Policy acquisition costs	247	245
Administrative expenses	60	68
Underwriting income (loss)	58	204
Net investment income (loss)	65	66
Other (income) expense	1	2
Amortization of purchased intangibles	3	3
Segment Income (loss)	119	265
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	183	157
Net premiums earned	110	94
Losses and loss expenses	85	65
Policy benefits	0	0
Policy acquisition costs	12	11
Administrative expenses	3	4
Underwriting income (loss)	10	14
Net investment income (loss)	7	9
Other (income) expense	0	0
Amortization of purchased intangibles	7	7
Segment Income (loss)	10	16
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,890	2,598
Net premiums earned	2,478	2,307
Losses and loss expenses	1,263	1,258
Policy benefits	0	0
Policy acquisition costs	668	642
Administrative expenses	266	258
Underwriting income (loss)	281	149
Net investment income (loss)	141	145
Other (income) expense	1	4
Amortization of purchased intangibles	12	12
Segment Income (loss)	409	278
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	207	218
Net premiums earned	180	186
Losses and loss expenses	120	87
Policy benefits	0	0
Policy acquisition costs	45	45
Administrative expenses	8	10
Underwriting income (loss)	7	44
Net investment income (loss)	70	69
Other (income) expense	0	0
Amortization of purchased intangibles	0	0
Segment Income (loss)	77	113
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	620	645
Net premiums earned	595	631
Losses and loss expenses	198	202
Policy benefits	163	185
Policy acquisition costs	179	180
Administrative expenses	82	76
Underwriting income (loss)	(27)	(12)
Net investment income (loss)	98	95
Other (income) expense	(34)	(12)
Amortization of purchased intangibles	1	1
Segment Income (loss)	104	94
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Net premiums written	0	0
Net premiums earned	0	0
Losses and loss expenses	9	11
Policy benefits	0	0
Policy acquisition costs	0	0
Administrative expenses	71	66
Underwriting income (loss)	(80)	(77)
Net investment income (loss)	(17)	(24)
Other (income) expense	(415)	23
Amortization of purchased intangibles	49	50
Segment Income (loss)	269	(174)
Realized Investment Gains (Losses)	888	(956)
Interest expense	122	132
Income tax expense	338	215
Net income	$ 697	$ (1,477)