Fair Value and Gross Unrealized Losses for Securities That Have Been in Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	$ 29,310	$ 30,654
Total Unrealized Losses	3,536	3,363
Less than 12 months Fair Value	10,115	10,582
Less than 12 months Unrealized Losses	523	435
12 months or longer Fair Value	19,195	20,072
12 months or longer Unrealized Losses	3,013	2,928
Banks/financial Institutions
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	14,000
Total Unrealized Losses	2,200
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	26,795	26,262
Total Unrealized Losses	2,902	2,798
Less than 12 months Fair Value	9,370	8,627
Less than 12 months Unrealized Losses	449	296
12 months or longer Fair Value	17,425	17,635
12 months or longer Unrealized Losses	2,453	2,502
Fixed Maturities | Japan government and agencies | Yen Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	940	1,634
Total Unrealized Losses	1	18
Less than 12 months Fair Value	859	1,634
Less than 12 months Unrealized Losses	1	18
12 months or longer Fair Value	81	0
12 months or longer Unrealized Losses	0	0
Fixed Maturities | Municipalities | Dollar Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	54	682
Total Unrealized Losses	8	42
Less than 12 months Fair Value	22	632
Less than 12 months Unrealized Losses	1	28
12 months or longer Fair Value	32	50
12 months or longer Unrealized Losses	7	14
Fixed Maturities | Municipalities | Yen Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	60	59
Total Unrealized Losses	4	2
Less than 12 months Fair Value	0	0
Less than 12 months Unrealized Losses	0	0
12 months or longer Fair Value	60	59
12 months or longer Unrealized Losses	4	2
Fixed Maturities | Mortgage- and asset-backed Securities | Dollar Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value		78
Total Unrealized Losses		13
Less than 12 months Fair Value		20
Less than 12 months Unrealized Losses		0
12 months or longer Fair Value		58
12 months or longer Unrealized Losses		13
Fixed Maturities | Mortgage- and asset-backed Securities | Yen Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	151	415
Total Unrealized Losses	1	15
Less than 12 months Fair Value	0	415
Less than 12 months Unrealized Losses	0	15
12 months or longer Fair Value	151	0
12 months or longer Unrealized Losses	1	0
Fixed Maturities | Public Utilities | Dollar Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	295	556
Total Unrealized Losses	27	36
Less than 12 months Fair Value	110	498
Less than 12 months Unrealized Losses	3	28
12 months or longer Fair Value	185	58
12 months or longer Unrealized Losses	24	8
Fixed Maturities | Public Utilities | Yen Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	4,995	2,877
Total Unrealized Losses	392	200
Less than 12 months Fair Value	2,404	766
Less than 12 months Unrealized Losses	141	47
12 months or longer Fair Value	2,591	2,111
12 months or longer Unrealized Losses	251	153
Fixed Maturities | Sovereign and Supranational | Dollar Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	66	45
Total Unrealized Losses	5	2
Less than 12 months Fair Value	34	12
Less than 12 months Unrealized Losses	2	0
12 months or longer Fair Value	32	33
12 months or longer Unrealized Losses	3	2
Fixed Maturities | Sovereign and Supranational | Yen Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	2,349	1,579
Total Unrealized Losses	199	77
Less than 12 months Fair Value	749	428
Less than 12 months Unrealized Losses	62	1
12 months or longer Fair Value	1,600	1,151
12 months or longer Unrealized Losses	137	76
Fixed Maturities | Banks/financial Institutions | Dollar Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	770	1,484
Total Unrealized Losses	121	108
Less than 12 months Fair Value	391	753
Less than 12 months Unrealized Losses	56	22
12 months or longer Fair Value	379	731
12 months or longer Unrealized Losses	65	86
Fixed Maturities | Banks/financial Institutions | Yen Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	10,175	10,609
Total Unrealized Losses	1,438	1,703
Less than 12 months Fair Value	1,639	1,506
Less than 12 months Unrealized Losses	46	40
12 months or longer Fair Value	8,536	9,103
12 months or longer Unrealized Losses	1,392	1,663
Fixed Maturities | Other Corporate | Dollar Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	834	1,741
Total Unrealized Losses	62	79
Less than 12 months Fair Value	639	1,456
Less than 12 months Unrealized Losses	27	52
12 months or longer Fair Value	195	285
12 months or longer Unrealized Losses	35	27
Fixed Maturities | Other Corporate | Yen Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	6,106	4,503
Total Unrealized Losses	644	503
Less than 12 months Fair Value	2,523	507
Less than 12 months Unrealized Losses	110	45
12 months or longer Fair Value	3,583	3,996
12 months or longer Unrealized Losses	534	458
Perpetual Securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	2,507	4,379
Total Unrealized Losses	633	563
Less than 12 months Fair Value	739	1,942
Less than 12 months Unrealized Losses	73	138
12 months or longer Fair Value	1,768	2,437
12 months or longer Unrealized Losses	560	425
Perpetual Securities | Dollar Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	217	208
Total Unrealized Losses	29	30
Less than 12 months Fair Value	109	149
Less than 12 months Unrealized Losses	4	19
12 months or longer Fair Value	108	59
12 months or longer Unrealized Losses	25	11
Perpetual Securities | Yen Denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	2,290	4,171
Total Unrealized Losses	604	533
Less than 12 months Fair Value	630	1,793
Less than 12 months Unrealized Losses	69	119
12 months or longer Fair Value	1,660	2,378
12 months or longer Unrealized Losses	535	414
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	8	13
Total Unrealized Losses	1	2
Less than 12 months Fair Value	6	13
Less than 12 months Unrealized Losses	1	1
12 months or longer Fair Value	2	0
12 months or longer Unrealized Losses	$ 0	$ 1