Derivative Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Derivative Financial Instruments, exposure
GMWB minimum account balance to make withdrawals by customer	$ 0
Cash and securities posted under collateral arrangements associated with derivative credit support agreements and Futures Commission Merchant agreements	369.7	296.9
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that were in a liability position	1,038.1	1,205.4
Collateral and initial margin posted supporting derivatives with credit-risk-related contingent features that were in a liability position	369.7	296.9
Additional collateral required to be posted if derivative credit-risk-related contingent features were triggered	69.2
Cash collateral received associated with derivative credit support annex agreements and Futures Commission Merchant agreements	30.0	207.8
Notional amount	34,938.3	34,256.4
Gross credit exposure	739.3	1,086.7
Less: collateral received	112.6	248.0
Net credit exposure	626.7	838.7
Interest rate swaps
Derivative Financial Instruments, exposure
Cash exchanged under contract	0
Principal payments made under contract	0
Notional amount	19,601.8	18,381.2
Gross credit exposure	477.1	683.9
Interest rate options
Derivative Financial Instruments, exposure
Notional amount	1,500.0	500.0
Gross credit exposure	29.3	48.5
Swaptions
Derivative Financial Instruments, exposure
Notional amount	325.0	325.0
Gross credit exposure	1.1	0.7
Interest rate futures
Derivative Financial Instruments, exposure
Notional amount	211.0	82.0
Currency swaps
Derivative Financial Instruments, exposure
Notional amount	2,559.3	3,454.1
Gross credit exposure	187.3	263.8
Currency forwards
Derivative Financial Instruments, exposure
Notional amount	264.0	557.2
Gross credit exposure	1.2	6.8
Currency options
Derivative Financial Instruments, exposure
Notional amount		1,400.0
Gross credit exposure		1.9
Equity options
Derivative Financial Instruments, exposure
Notional amount	1,905.3	1,811.8
Gross credit exposure	36.1	74.3
Equity futures
Derivative Financial Instruments, exposure
Notional amount	278.7	373.6
Credit default swaps
Derivative Financial Instruments, exposure
Notional amount	1,078.2	1,378.3
Gross credit exposure	7.2	6.8
Total return swaps
Derivative Financial Instruments, exposure
Notional amount	100.0	100.0
Embedded derivative financial instruments
Derivative Financial Instruments, exposure
Notional amount	$ 7,115.0	$ 5,893.2