Fair Value Hierarchy Levels of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) (Fair Value, Measurements, Recurring, USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Assets:
Swaps	$ 407	$ 564
Total assets	57,836	56,454
Liabilities:
Total liabilities	608	741
Municipalities | Level 1 | Fixed Maturities
Assets:
Securities	0	0
Municipalities | Level 2 | Fixed Maturities
Assets:
Securities	1,154	973
Municipalities | Level 3 | Fixed Maturities
Assets:
Securities	0	0
Municipalities | Fixed Maturities
Assets:
Securities	1,154	973
Mortgage- and asset-backed Securities | Level 1 | Fixed Maturities
Assets:
Securities	0	0
Mortgage- and asset-backed Securities | Level 2 | Fixed Maturities
Assets:
Securities	1,179	1,539
Mortgage- and asset-backed Securities | Level 3 | Fixed Maturities
Assets:
Securities	268	267
Mortgage- and asset-backed Securities | Fixed Maturities
Assets:
Securities	1,447	1,806
Public Utilities | Level 1 | Fixed Maturities
Assets:
Securities	0	0
Public Utilities | Level 2 | Fixed Maturities
Assets:
Securities	6,252	5,369
Public Utilities | Level 3 | Fixed Maturities
Assets:
Securities	0	0
Public Utilities | Fixed Maturities
Assets:
Securities	6,252	5,369
Sovereign and Supranational | Level 1 | Fixed Maturities
Assets:
Securities	0	0
Sovereign and Supranational | Level 2 | Fixed Maturities
Assets:
Securities	2,343	1,394
Sovereign and Supranational | Level 3 | Fixed Maturities
Assets:
Securities	0	0
Sovereign and Supranational | Fixed Maturities
Assets:
Securities	2,343	1,394
Banks/financial Institutions | Level 1 | Fixed Maturities
Assets:
Securities	0	0
Banks/financial Institutions | Level 1 | Perpetual Securities
Assets:
Securities	0	0
Banks/financial Institutions | Level 2 | Fixed Maturities
Assets:
Securities	7,623	8,047
Banks/financial Institutions | Level 2 | Perpetual Securities
Assets:
Securities	6,213	7,169
Banks/financial Institutions | Level 3 | Fixed Maturities
Assets:
Securities	25	386
Banks/financial Institutions | Level 3 | Perpetual Securities
Assets:
Securities	0	0
Banks/financial Institutions | Fixed Maturities
Assets:
Securities	7,648	8,433
Banks/financial Institutions | Perpetual Securities
Assets:
Securities	6,213	7,169
Other Corporate | Level 1 | Fixed Maturities
Assets:
Securities	0	0
Other Corporate | Level 1 | Perpetual Securities
Assets:
Securities	0	0
Other Corporate | Level 2 | Fixed Maturities
Assets:
Securities	15,818	13,162
Other Corporate | Level 2 | Perpetual Securities
Assets:
Securities	317	343
Other Corporate | Level 3 | Fixed Maturities
Assets:
Securities	0	0
Other Corporate | Level 3 | Perpetual Securities
Assets:
Securities	0	0
Other Corporate | Fixed Maturities
Assets:
Securities	15,818	13,162
Other Corporate | Perpetual Securities
Assets:
Securities	317	343
Collateralized Debt Obligations | Level 1 | Fixed Maturities
Assets:
Securities	0	0
Collateralized Debt Obligations | Level 2 | Fixed Maturities
Assets:
Securities	0	0
Collateralized Debt Obligations | Level 3 | Fixed Maturities
Assets:
Securities	0	5
Collateralized Debt Obligations | Fixed Maturities
Assets:
Securities	0	5
Interest rate swaps
Assets:
Swaps	99	124
Liabilities:
Swaps	40	70
Interest rate swaps | Level 1
Assets:
Swaps	0	0
Liabilities:
Swaps	0	0
Interest rate swaps | Level 2
Assets:
Swaps	0	124
Liabilities:
Swaps	0	70
Interest rate swaps | Level 3
Assets:
Swaps	99	0
Liabilities:
Swaps	40	0
Foreign currency swaps
Assets:
Swaps	308	440
Liabilities:
Swaps	374	328
Foreign currency swaps | Level 1
Assets:
Swaps	0	0
Liabilities:
Swaps	0	0
Foreign currency swaps | Level 2
Assets:
Swaps	0	151
Liabilities:
Swaps	0	280
Foreign currency swaps | Level 3
Assets:
Swaps	308	289
Liabilities:
Swaps	374	48
Credit default swaps
Liabilities:
Swaps	194	343
Credit default swaps | Level 1
Liabilities:
Swaps	0	0
Credit default swaps | Level 2
Liabilities:
Swaps	0	0
Credit default swaps | Level 3
Liabilities:
Swaps	194	343
Level 1
Assets:
Swaps	0	0
Total assets	15,500	16,548
Liabilities:
Total liabilities	0	0
Level 1 | Fixed Maturities
Assets:
Securities	15,484	16,534
Level 1 | Fixed Maturities | Government and Agencies Debt Securities
Assets:
Securities	15,484	16,534
Level 1 | Perpetual Securities
Assets:
Securities	0	0
Level 1 | Equity Securities
Assets:
Securities	16	14
Level 2
Assets:
Swaps	0	275
Total assets	41,632	38,955
Liabilities:
Total liabilities	0	350
Level 2 | Fixed Maturities
Assets:
Securities	35,098	31,163
Level 2 | Fixed Maturities | Government and Agencies Debt Securities
Assets:
Securities	729	679
Level 2 | Perpetual Securities
Assets:
Securities	6,530	7,512
Level 2 | Equity Securities
Assets:
Securities	4	5
Level 3
Assets:
Swaps	407	289
Total assets	704	951
Liabilities:
Total liabilities	608	391
Level 3 | Fixed Maturities
Assets:
Securities	293	658
Level 3 | Fixed Maturities | Government and Agencies Debt Securities
Assets:
Securities	0	0
Level 3 | Perpetual Securities
Assets:
Securities	0	0
Level 3 | Equity Securities
Assets:
Securities	4	4
Fixed Maturities
Assets:
Securities	50,875	48,355
Fixed Maturities | Government and Agencies Debt Securities
Assets:
Securities	16,213	17,213
Perpetual Securities
Assets:
Securities	6,530	7,512
Equity Securities
Assets:
Securities	$ 24	$ 23