Fair Value Measurements (Details) - Narrative (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale fixed income securities, Fair Value	$ 27,846,920,000	$ 26,190,025,000
Available for sale fixed income securities, Amortized Cost	26,544,806,000	25,771,715,000
Fair value of securities - AFS	28,818,982,000	27,017,285,000
Derivative, Collateral, Obligation to Return Cash		77,100,000
Derivative Cash Collateral Paid	12,100,000
Other investments - structured transactions	312,122,000	323,705,000
Spread on US Treasury rate for determining discount rate for fair value of deposit liabilities	1.003%	1.618%
Corporate [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale fixed income securities, Fair Value	10,482,000,000	10,108,622,000
Available for sale fixed income securities, Amortized Cost	9,837,962,000	9,866,677,000
Medium-term Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale fixed income securities, Fair Value	194,300,000	266,000,000
Available for sale fixed income securities, Amortized Cost	194,800,000	297,700,000
Other asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale fixed income securities, Fair Value	1,470,711,000	1,340,249,000
Available for sale fixed income securities, Amortized Cost	1,426,483,000	1,341,309,000
Covered Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale fixed income securities, Fair Value	647,100,000	353,900,000
Available for sale fixed income securities, Amortized Cost	605,400,000	345,400,000
Fixed Income Funds Member
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of securities - AFS	101,900,000	91,600,000
Collateral and counterparty netting
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative, Collateral, Obligation to Return Cash		77,100,000
Derivative Cash Collateral Paid	$ 12,100,000