Fair Value Measurements - Fair Value Measurement (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale	$ 276.6		$ 314.4
Derivatives	1,044.7		1,149.3
Derivatives	1,013.5		1,351.8
Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	4,677.1		4,617.0
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	3,834.2		4,396.9
Assets held in separate accounts	102,600.4		101,437.5
Total assets	111,332.3		110,691.8
Percentage of Level to total	59.30%		59.70%
Total liabilities	58.0		20.9
Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	69,247.9		67,318.8
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	270.4		44.9
Assets held in separate accounts	5,221.7		5,376.5
Total assets	75,704.1		73,813.4
Percentage of Level to total	40.30%		39.80%
Total liabilities	1,039.0		1,395.4
Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	878.1		783.0
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	0		0
Assets held in separate accounts	17.9		13.1
Total assets	1,032.6		946.2
Percentage of Level to total	0.40%		0.50%
Total liabilities	2,981.7		2,660.1
Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	74,803.1		72,718.8
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	4,104.6		4,441.8
Assets held in separate accounts	107,840.0		106,827.1
Total assets	188,069.0		185,451.4
Percentage of Level to total	100.00%		100.00%
Total liabilities	4,078.7		4,076.4
Interest rate contracts | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Derivatives	0		0
Interest rate contracts | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	765.2		912.0
Derivatives	867.2		1,239.5
Interest rate contracts | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Derivatives	0		0
Interest rate contracts | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	765.2		912.0
Derivatives	867.2		1,239.5
Foreign exchange contracts | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Derivatives	0		0
Foreign exchange contracts | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	26.6		24.1
Derivatives	31.3		44.9
Foreign exchange contracts | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Derivatives	0		0
Foreign exchange contracts | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	26.6		24.1
Derivatives	31.3		44.9
Equity contracts | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	5.5		1.9
Derivatives	58.0		20.9
Equity contracts | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	138.9		83.3
Derivatives	44.6		32.0
Equity contracts | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	73.2		87.5
Derivatives	0		0
Equity contracts | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	217.6		172.7
Derivatives	102.6		52.9
Credit contracts | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Derivatives	0		0
Credit contracts | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	29.0		33.2
Derivatives	0		0
Credit contracts | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	6.3		7.3
Derivatives	12.4		14.5
Credit contracts | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	35.3		40.5
Derivatives	12.4		14.5
Reinsurance agreements | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Reinsurance agreements | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	95.9		79.0
Reinsurance agreements | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Reinsurance agreements | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	95.9		79.0
FIA | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0		0
FIA | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0		0
FIA | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	1,808.0		1,736.7
FIA | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	1,808.0		1,736.7
GMAB/GMWB/GMWBL | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0	[1]	0
GMAB/GMWB/GMWBL | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0	[1]	0
GMAB/GMWB/GMWBL | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	1,143.3	[1]	908.9
GMAB/GMWB/GMWBL | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	1,143.3	[1]	908.9
Stabilizer and MCGs | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0		0
Stabilizer and MCGs | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0		0
Stabilizer and MCGs | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	18.0		0
Stabilizer and MCGs | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	18.0		0
Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	74,803.1		72,718.8
U.S. Treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	5,263.1		5,181.2
U.S. Treasuries | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	4,677.1		4,617.0
U.S. Treasuries | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	586.0		564.2
U.S. Treasuries | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	0		0
U.S. Treasuries | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	5,263.1		5,181.2
U.S. government agencies and authorities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	644.1		618.9
U.S. government agencies and authorities | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	0		0
U.S. government agencies and authorities | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	636.9		604.5
U.S. government agencies and authorities | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	7.2		14.4
U.S. government agencies and authorities | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	644.1		618.9
U.S. corporate, state and municipalities | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	0		0
U.S. corporate, state and municipalities | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	38,898.4		37,303.2
U.S. corporate, state and municipalities | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	568.7		456.5
U.S. corporate, state and municipalities | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	39,467.1		37,759.7
Foreign
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	907.2	[2]	1,051.4	[2]
Foreign | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	0	[2]	0	[2]
Foreign | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	16,609.9	[2]	16,202.2	[2]
Foreign | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	130.7	[2]	154.3	[2]
Foreign | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	16,740.6	[2]	16,356.5	[2]
Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	7,052.7		7,123.7
Residential mortgage-backed | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	0		0
Residential mortgage-backed | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	6,948.3		7,025.1
Residential mortgage-backed | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	104.4		98.6
Residential mortgage-backed | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	7,052.7		7,123.7
Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	3,796.1		3,752.1
Commercial mortgage-backed | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	0		0
Commercial mortgage-backed | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	3,771.2		3,752.1
Commercial mortgage-backed | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	24.9		0
Commercial mortgage-backed | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	3,796.1		3,752.1
Other asset-backed securities | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	0		0
Other asset-backed securities | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	1,797.2		1,867.5
Other asset-backed securities | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	42.2		59.2
Other asset-backed securities | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	1,839.4		1,926.7
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale	276.6		314.4
Equity securities | Measured at fair value on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale	215.1		238.5
Equity securities | Measured at fair value on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale	4.4		20.6
Equity securities | Measured at fair value on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale	57.1		55.3
Equity securities | Measured at fair value on a recurring basis | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale	$ 276.6		$ 314.4

[1] Guaranteed minimum accumulation benefits ("GMAB"), Guaranteed minimum withdrawal benefits ("GMWB") and Guaranteed minimum withdrawal benefits with life payouts ("GMWBL").

[2] Primarily U.S. dollar denominated.