Derivative Instruments (Balance Sheet Offsetting) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative Instruments [Abstract]
Derivative Instruments, Gross amount of recognized assets	$ 3,022	$ 3,547
Derivative Instruments, Gross amounts offset	(754)	(895)
Derivative Instruments, Net amount of assets	2,268	2,652
Derivative Instruments, Cash collateral received	(2,191)	(2,624)
Derivative Instruments, Net amount	77	28
Embedded Derivative Instruments, Gross amount of recognized assets
Embedded Derivative Instruments, Gross amounts offset
Embedded Derivative Instruments, Net amount of assets
Embedded Derivative Instruments, Cash collateral received
Embedded Derivative Instruments, Net amount
Securities Lending and Reverse Repurchase Agreements, Gross amount of recognized assets
Securities Lending and Reverse Repurchase Agreements, Gross amounts offset
Securities Lending and Reverse Repurchase Agreements, Net amount of assets
Securities Lending and Reverse Repurchase Agreements, Cash collateral received
Securities Lending and Reverse Repurchase Agreements, Net amount
Total, Gross amount of recognized assets	3,022	3,547
Total, Gross amounts offset	(754)	(895)
Total, Net amount of assets	2,268	2,652
Total, Cash collateral received	(2,191)	(2,624)
Total, Net amount	77	28
Derivative Instruments, Gross amount of recognized liabilities	764	906
Derivative Instruments, Gross amounts offset	(754)	(895)
Derivative Instruments, Net amount of liabilities	10	11
Derivative Instruments, Financial instruments
Derivative Instruments, Net amount	10	11
Embedded Derivative Instruments, Gross amount of recognized liabilities	1,251	1,856
Embedded Derivative Instruments, Gross amounts offset
Embedded Derivative Instruments, Net amount of liabilities	1,251	1,856
Embedded Derivative Instruments, Financial instruments
Embedded Derivative Instruments, Net amount	1,251	1,856
Securities Lending and Reverse Repurchase Agreements, Gross amount of recognized liabilities	2,115	1,614
Securities Lending and Reverse Repurchase Agreements, Gross amounts offset
Securities Lending and Reverse Repurchase Agreements, Net amount of liabilities	2,115	1,614
Securities Lending and Reverse Repurchase Agreements, Financial instruments	(2,115)	(1,614)
Securities Lending and Reverse Repurchase Agreements, Net amount
Total, Gross amount of recognized liabilities	4,130	4,376
Total, Gross amounts offset	(754)	(895)
Total, Net amount of liabilities	3,376	3,481
Total, Financial instruments	(2,115)	(1,614)
Total, Net amount	$ 1,261	$ 1,867