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Investments in Debt Securities, Perpetual Securities and Equity Securities (Detail) (USD $)
In Millions
Jun. 30, 2011
Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost $ 58,548 $ 55,951
Gross Unrealized Gains 2,712 2,539
Gross Unrealized Losses 1,628 2,600
Fair Value 59,632 55,890
Cost or Amortized Cost 31,908 30,084
Gross Unrealized Gains 976 1,148
Gross Unrealized Losses 902 763
Fair Value 31,982 30,469
Fixed Maturities
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 51,268 48,102
Gross Unrealized Gains 2,467 2,288
Gross Unrealized Losses 1,229 2,035
Fair Value 52,506 48,355
Fixed Maturities | Yen Denominated
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 33,281 32,948
Gross Unrealized Gains 1,066 1,071
Gross Unrealized Losses 1,048 1,755
Fair Value 33,299 32,264
Cost or Amortized Cost 31,908 30,084
Gross Unrealized Gains 976 1,148
Gross Unrealized Losses 902 763
Fair Value 31,982 30,469
Fixed Maturities | Yen Denominated | Japan government and agencies
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 17,436 16,607
Gross Unrealized Gains 556 584
Gross Unrealized Losses 12 14
Fair Value 17,980 17,177
Cost or Amortized Cost 3,216 344
Gross Unrealized Gains 70 4
Gross Unrealized Losses 4 4
Fair Value 3,282 344
Fixed Maturities | Yen Denominated | Mortgage- and asset-backed Securities
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 923 1,224
Gross Unrealized Gains 32 35
Gross Unrealized Losses 5 15
Fair Value 950 1,244
Cost or Amortized Cost 136 146
Gross Unrealized Gains 4 5
Gross Unrealized Losses 0 0
Fair Value 140 151
Fixed Maturities | Yen Denominated | Public Utilities
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 2,575 2,554
Gross Unrealized Gains 71 117
Gross Unrealized Losses 117 80
Fair Value 2,529 2,591
Cost or Amortized Cost 6,396 6,339
Gross Unrealized Gains 260 326
Gross Unrealized Losses 127 120
Fair Value 6,529 6,545
Fixed Maturities | Yen Denominated | Sovereign and Supranational
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 1,721 903
Gross Unrealized Gains 119 47
Gross Unrealized Losses 19 12
Fair Value 1,821 938
Cost or Amortized Cost 4,173 4,951
Gross Unrealized Gains 198 305
Gross Unrealized Losses 89 65
Fair Value 4,282 5,191
Fixed Maturities | Yen Denominated | Banks/financial Institutions
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 4,766 5,927
Gross Unrealized Gains 134 152
Gross Unrealized Losses 410 1,177
Fair Value 4,490 4,902
Cost or Amortized Cost 12,303 12,618
Gross Unrealized Gains 211 216
Gross Unrealized Losses 589 526
Fair Value 11,925 12,308
Fixed Maturities | Yen Denominated | Other Corporate
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 5,860 5,733
Gross Unrealized Gains 154 136
Gross Unrealized Losses 485 457
Fair Value 5,529 5,412
Cost or Amortized Cost 5,149 5,279
Gross Unrealized Gains 214 274
Gross Unrealized Losses 89 46
Fair Value 5,274 5,507
Fixed Maturities | Yen Denominated | Municipalities
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 535 407
Gross Unrealized Gains 19 18
Gross Unrealized Losses 4 2
Fair Value 550 423
Fixed Maturities | Dollar Denominated
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 17,987 15,154
Gross Unrealized Gains 1,401 1,217
Gross Unrealized Losses 181 280
Fair Value 19,207 16,091
Fixed Maturities | Dollar Denominated | Mortgage- and asset-backed Securities
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 451 [1] 485 [2]
Gross Unrealized Gains 98 [1] 90 [2]
Gross Unrealized Losses 1 [1] 13 [2]
Fair Value 548 [1] 562 [2]
Fixed Maturities | Dollar Denominated | Public Utilities
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 2,824 2,568
Gross Unrealized Gains 266 246
Gross Unrealized Losses 35 36
Fair Value 3,055 2,778
Fixed Maturities | Dollar Denominated | Sovereign and Supranational
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 392 395
Gross Unrealized Gains 67 63
Gross Unrealized Losses 3 2
Fair Value 456 456
Fixed Maturities | Dollar Denominated | Banks/financial Institutions
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 3,298 3,496
Gross Unrealized Gains 180 143
Gross Unrealized Losses 52 108
Fair Value 3,426 3,531
Fixed Maturities | Dollar Denominated | Other Corporate
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 8,175 7,167
Gross Unrealized Gains 700 662
Gross Unrealized Losses 69 79
Fair Value 8,806 7,750
Fixed Maturities | Dollar Denominated | U.S. Government and Agencies
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 1,816 32
Gross Unrealized Gains 71 4
Gross Unrealized Losses 0 0
Fair Value 1,887 36
Fixed Maturities | Dollar Denominated | Municipalities
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 1,027 1,006
Gross Unrealized Gains 19 9
Gross Unrealized Losses 21 42
Fair Value 1,025 973
Fixed Maturities | Dollar Denominated | Collateralized Debt Obligations
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 4 5
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 4 5
Perpetual Securities
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 7,259 7,827
Gross Unrealized Gains 241 248
Gross Unrealized Losses 397 563
Fair Value 7,103 7,512
Perpetual Securities | Yen Denominated | Banks/financial Institutions
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 6,570 7,080
Gross Unrealized Gains 200 172
Gross Unrealized Losses 374 533
Fair Value 6,396 6,719
Perpetual Securities | Yen Denominated | Other Corporate
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 332 328
Gross Unrealized Gains 25 15
Gross Unrealized Losses 0 0
Fair Value 357 343
Perpetual Securities | Dollar Denominated | Banks/financial Institutions
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 357 419
Gross Unrealized Gains 16 61
Gross Unrealized Losses 23 30
Fair Value 350 450
Equity Securities
    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 21 22
Gross Unrealized Gains 4 3
Gross Unrealized Losses 2 2
Fair Value $ 23 $ 23
[1] Includes less than $1 million of other-than-temporary non-credit-related losses
[2] Includes $4 of other-than-temporary non-credit-related losses