Fair Value Measurements - Fair Value Measurement (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale	$ 270.5		$ 314.4
Derivatives	1,136.8		1,149.3
Other investments	106.8		124.6
Derivatives	656.8		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	3,606.5		4,617.0
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	3,555.5		4,396.9
Assets held in separate accounts	102,248.3		101,437.5
Total assets	109,640.1		110,691.8
Percentage of Level to total	58.60%		59.70%
Total liabilities	1.6		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	69,500.1		67,318.8
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	119.9		44.9
Assets held in separate accounts	4,797.3		5,376.5
Total assets	75,442.4		73,813.4
Percentage of Level to total	40.40%		39.80%
Total liabilities	754.7		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	1,628.5		783.0
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	0		0
Assets held in separate accounts	13.7		13.1
Total assets	1,794.6		946.2
Percentage of Level to total	1.00%		0.50%
Total liabilities	3,120.4		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	74,735.1		72,718.8
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	3,675.4		4,441.8
Assets held in separate accounts	107,059.3		106,827.1
Total assets	186,877.1		185,451.4
Percentage of Level to total	100.00%		100.00%
Total liabilities	3,876.7		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	796.2		912.0
Derivatives	570.1		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	796.2		912.0
Derivatives	570.1		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	47.5		24.1
Derivatives	20.5		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	47.5		24.1
Derivatives	20.5		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	16.7		1.9
Derivatives	1.6		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	152.1		83.3
Derivatives	40.6		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	82.9		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	251.7		172.7
Derivatives	42.2		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	29.3		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	12.1		7.3
Derivatives	24.0		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	41.4		40.5
Derivatives	24.0		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	123.5		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	123.5		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	1,894.2		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	1,894.2		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	1,157.2	[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	1,157.2		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	45.0		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	45.0		0
Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	74,735.1		72,718.8
U.S. Treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	4,187.1		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	3,606.5		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	580.6		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	4,187.1		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	462.3		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	462.3		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	0		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	462.3		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	39,405.0		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	1,128.0		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	40,533.0		37,759.7
Foreign
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	897.0	[2]	1,051.4
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	16,642.1	[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	375.4	[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	17,017.5		16,356.5
Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	6,696.4		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	6,603.5		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	92.9		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	6,696.4		7,123.7
Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged	4,030.8		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	4,008.9		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	21.9		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	4,030.8		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	1,797.7		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	10.3		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	1,808.0		1,926.7
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale	270.5		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	213.1		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	0		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	57.4		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	$ 270.5		$ 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.