Investments in Debt Securities, Perpetual Securities and Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 52,275	$ 55,951
Gross Unrealized Gains	3,679	2,539
Gross Unrealized Losses	1,918	2,600
Fair Value	54,036	55,890
Cost or Amortized Cost	47,009	30,084
Gross Unrealized Gains	992	1,148
Gross Unrealized Losses	1,618	763
Fair Value	46,383	30,469
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	45,356	48,102
Gross Unrealized Gains	3,500	2,288
Gross Unrealized Losses	1,284	2,035
Fair Value	47,572	48,355
Fixed Maturities | Yen Denominated
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	28,099	32,948
Gross Unrealized Gains	1,053	1,071
Gross Unrealized Losses	1,061	1,755
Fair Value	28,091	32,264
Cost or Amortized Cost	47,009	30,084
Gross Unrealized Gains	992	1,148
Gross Unrealized Losses	1,618	763
Fair Value	46,383	30,469
Fixed Maturities | Yen Denominated | Japan government and agencies
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	11,108	16,607
Gross Unrealized Gains	670	584
Gross Unrealized Losses	0	14
Fair Value	11,778	17,177
Cost or Amortized Cost	18,775	344
Gross Unrealized Gains	297	4
Gross Unrealized Losses	1	4
Fair Value	19,071	344
Fixed Maturities | Yen Denominated | Municipalities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	553	407
Gross Unrealized Gains	35	18
Gross Unrealized Losses	4	2
Fair Value	584	423
Fixed Maturities | Yen Denominated | Mortgage- and asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	912	1,224
Gross Unrealized Gains	43	35
Gross Unrealized Losses	1	15
Fair Value	954	1,244
Cost or Amortized Cost	129	146
Gross Unrealized Gains	5	5
Gross Unrealized Losses	0	0
Fair Value	134	151
Fixed Maturities | Yen Denominated | Public Utilities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,850	2,554
Gross Unrealized Gains	59	117
Gross Unrealized Losses	226	80
Fair Value	3,683	2,591
Cost or Amortized Cost	5,615	6,339
Gross Unrealized Gains	188	326
Gross Unrealized Losses	166	120
Fair Value	5,637	6,545
Fixed Maturities | Yen Denominated | Sovereign and Supranational
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,704	903
Gross Unrealized Gains	87	47
Gross Unrealized Losses	16	12
Fair Value	1,775	938
Cost or Amortized Cost	4,200	4,951
Gross Unrealized Gains	148	305
Gross Unrealized Losses	183	65
Fair Value	4,165	5,191
Fixed Maturities | Yen Denominated | Banks/financial Institutions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	4,312	5,927
Gross Unrealized Gains	74	152
Gross Unrealized Losses	359	1,177
Fair Value	4,027	4,902
Cost or Amortized Cost	12,389	12,618
Gross Unrealized Gains	170	216
Gross Unrealized Losses	1,079	526
Fair Value	11,480	12,308
Fixed Maturities | Yen Denominated | Other Corporate
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	6,213	5,733
Gross Unrealized Gains	120	136
Gross Unrealized Losses	459	457
Fair Value	5,874	5,412
Cost or Amortized Cost	5,348	5,279
Gross Unrealized Gains	149	274
Gross Unrealized Losses	185	46
Fair Value	5,312	5,507
Fixed Maturities | Dollar Denominated
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	17,257	15,154
Gross Unrealized Gains	2,447	1,217
Gross Unrealized Losses	223	280
Fair Value	19,481	16,091
Fixed Maturities | Dollar Denominated | Municipalities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,060	1,006
Gross Unrealized Gains	107	9
Gross Unrealized Losses	8	42
Fair Value	1,159	973
Fixed Maturities | Dollar Denominated | Mortgage- and asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	310	485	[1]
Gross Unrealized Gains	74	90	[1]
Gross Unrealized Losses	0	13	[1]
Fair Value	384	562	[1]
Fixed Maturities | Dollar Denominated | Public Utilities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,052	2,568
Gross Unrealized Gains	517	246
Gross Unrealized Losses	27	36
Fair Value	3,542	2,778
Fixed Maturities | Dollar Denominated | Sovereign and Supranational
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	449	395
Gross Unrealized Gains	89	63
Gross Unrealized Losses	5	2
Fair Value	533	456
Fixed Maturities | Dollar Denominated | Banks/financial Institutions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,324	3,496
Gross Unrealized Gains	223	143
Gross Unrealized Losses	121	108
Fair Value	3,426	3,531
Fixed Maturities | Dollar Denominated | Other Corporate
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	9,031	7,167
Gross Unrealized Gains	1,433	662
Gross Unrealized Losses	62	79
Fair Value	10,402	7,750
Fixed Maturities | Dollar Denominated | U.S. Government and Agencies
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	31	32
Gross Unrealized Gains	4	4
Gross Unrealized Losses	0	0
Fair Value	35	36
Fixed Maturities | Dollar Denominated | Collateralized Debt Obligations
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		5
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		5
Perpetual Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	6,897	7,827
Gross Unrealized Gains	175	248
Gross Unrealized Losses	633	563
Fair Value	6,439	7,512
Perpetual Securities | Yen Denominated | Banks/financial Institutions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	6,217	7,080
Gross Unrealized Gains	155	172
Gross Unrealized Losses	604	533
Fair Value	5,768	6,719
Perpetual Securities | Yen Denominated | Other Corporate
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	344	328
Gross Unrealized Gains	17	15
Gross Unrealized Losses	0	0
Fair Value	361	343
Perpetual Securities | Dollar Denominated | Banks/financial Institutions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	336	419
Gross Unrealized Gains	3	61
Gross Unrealized Losses	29	30
Fair Value	310	450
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	22	22
Gross Unrealized Gains	4	3
Gross Unrealized Losses	1	2
Fair Value	$ 25	$ 23

[1] Includes $4 of other-than-temporary non-credit-related losses