Fair Value Hierarchy Levels of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Fair Value, Measurements, Recurring, USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Assets:
Swaps	$ 375	$ 564
Total assets	54,411	56,454
Liabilities:
Total liabilities	531	741
Interest rate swaps
Assets:
Swaps	34	124
Liabilities:
Swaps	4	70
Foreign currency swaps
Assets:
Swaps	341	440
Liabilities:
Swaps	397	328
Credit default swaps
Liabilities:
Swaps	130	343
Fixed Maturities
Assets:
Securities	47,572	48,355
Fixed Maturities | Government and Agencies Debt Securities
Assets:
Securities	11,813	17,213
Fixed Maturities | Municipalities
Assets:
Securities	1,159	973
Fixed Maturities | Mortgage- and asset-backed Securities
Assets:
Securities	1,338	1,806
Fixed Maturities | Public Utilities
Assets:
Securities	7,225	5,369
Fixed Maturities | Collateralized Debt Obligations
Assets:
Securities	0	5
Fixed Maturities | Sovereign and Supranational
Assets:
Securities	2,308	1,394
Fixed Maturities | Banks/financial Institutions
Assets:
Securities	7,453	8,433
Fixed Maturities | Other Corporate
Assets:
Securities	16,276	13,162
Perpetual Securities
Assets:
Securities	6,439	7,512
Perpetual Securities | Banks/financial Institutions
Assets:
Securities	6,078	7,169
Perpetual Securities | Other Corporate
Assets:
Securities	361	343
Equity Securities
Assets:
Securities	25	23
Level 1
Assets:
Swaps	0	0
Total assets	11,107	16,548
Liabilities:
Total liabilities	0	0
Level 1 | Interest rate swaps
Assets:
Swaps	0	0
Liabilities:
Swaps	0	0
Level 1 | Foreign currency swaps
Assets:
Swaps	0	0
Liabilities:
Swaps	0	0
Level 1 | Credit default swaps
Liabilities:
Swaps	0	0
Level 1 | Fixed Maturities
Assets:
Securities	11,092	16,534
Level 1 | Fixed Maturities | Government and Agencies Debt Securities
Assets:
Securities	11,092	16,534
Level 1 | Fixed Maturities | Municipalities
Assets:
Securities	0	0
Level 1 | Fixed Maturities | Mortgage- and asset-backed Securities
Assets:
Securities	0	0
Level 1 | Fixed Maturities | Public Utilities
Assets:
Securities	0	0
Level 1 | Fixed Maturities | Collateralized Debt Obligations
Assets:
Securities	0	0
Level 1 | Fixed Maturities | Sovereign and Supranational
Assets:
Securities	0	0
Level 1 | Fixed Maturities | Banks/financial Institutions
Assets:
Securities	0	0
Level 1 | Fixed Maturities | Other Corporate
Assets:
Securities	0	0
Level 1 | Perpetual Securities
Assets:
Securities	0	0
Level 1 | Perpetual Securities | Banks/financial Institutions
Assets:
Securities	0	0
Level 1 | Perpetual Securities | Other Corporate
Assets:
Securities	0	0
Level 1 | Equity Securities
Assets:
Securities	15	14
Level 2
Assets:
Swaps	0	275
Total assets	38,970	38,955
Liabilities:
Total liabilities	0	350
Level 2 | Interest rate swaps
Assets:
Swaps	0	124
Liabilities:
Swaps	0	70
Level 2 | Foreign currency swaps
Assets:
Swaps	0	151
Liabilities:
Swaps	0	280
Level 2 | Credit default swaps
Liabilities:
Swaps	0	0
Level 2 | Fixed Maturities
Assets:
Securities	33,051	31,163
Level 2 | Fixed Maturities | Government and Agencies Debt Securities
Assets:
Securities	721	679
Level 2 | Fixed Maturities | Municipalities
Assets:
Securities	1,159	973
Level 2 | Fixed Maturities | Mortgage- and asset-backed Securities
Assets:
Securities	944	1,539
Level 2 | Fixed Maturities | Public Utilities
Assets:
Securities	6,803	5,369
Level 2 | Fixed Maturities | Collateralized Debt Obligations
Assets:
Securities	0	0
Level 2 | Fixed Maturities | Sovereign and Supranational
Assets:
Securities	1,874	1,394
Level 2 | Fixed Maturities | Banks/financial Institutions
Assets:
Securities	6,379	8,047
Level 2 | Fixed Maturities | Other Corporate
Assets:
Securities	15,171	13,162
Level 2 | Perpetual Securities
Assets:
Securities	5,913	7,512
Level 2 | Perpetual Securities | Banks/financial Institutions
Assets:
Securities	5,552	7,169
Level 2 | Perpetual Securities | Other Corporate
Assets:
Securities	361	343
Level 2 | Equity Securities
Assets:
Securities	6	5
Level 3
Assets:
Swaps	375	289
Total assets	4,334	951
Liabilities:
Total liabilities	531	391
Level 3 | Interest rate swaps
Assets:
Swaps	34	0
Liabilities:
Swaps	4	0
Level 3 | Foreign currency swaps
Assets:
Swaps	341	289
Liabilities:
Swaps	397	48
Level 3 | Credit default swaps
Liabilities:
Swaps	130	343
Level 3 | Fixed Maturities
Assets:
Securities	3,429	658
Level 3 | Fixed Maturities | Government and Agencies Debt Securities
Assets:
Securities	0	0
Level 3 | Fixed Maturities | Municipalities
Assets:
Securities	0	0
Level 3 | Fixed Maturities | Mortgage- and asset-backed Securities
Assets:
Securities	394	267
Level 3 | Fixed Maturities | Public Utilities
Assets:
Securities	422	0
Level 3 | Fixed Maturities | Collateralized Debt Obligations
Assets:
Securities	0	5
Level 3 | Fixed Maturities | Sovereign and Supranational
Assets:
Securities	434	0
Level 3 | Fixed Maturities | Banks/financial Institutions
Assets:
Securities	1,074	386
Level 3 | Fixed Maturities | Other Corporate
Assets:
Securities	1,105	0
Level 3 | Perpetual Securities
Assets:
Securities	526	0
Level 3 | Perpetual Securities | Banks/financial Institutions
Assets:
Securities	526	0
Level 3 | Perpetual Securities | Other Corporate
Assets:
Securities	0	0
Level 3 | Equity Securities
Assets:
Securities	$ 4	$ 4