Fair Value Measurements - Fair Value Measurement (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets:
Equity securities, available-for-sale	$ 123.6	$ 121.9
Derivatives	657.3	562.0
Liabilities:
Derivatives	298.8	217.0
Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged, Fair Value	22,222.1	21,616.6
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	848.7	1,048.1
Assets held in separate accounts	64,218.8	62,808.1
Total assets, fair value	88,070.5	86,156.7
Liabilities:
Total liabilities, fair value	475.4	333.2
Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged, Fair Value	748.8	712.9
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	793.3	1,046.6
Assets held in separate accounts	58,952.1	57,492.6
Total assets, fair value	60,579.8	59,337.4
Liabilities:
Total liabilities, fair value	0.1	0.1
Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged, Fair Value	20,879.3	20,324.5
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	53.9	0
Assets held in separate accounts	5,266.0	5,313.1
Total assets, fair value	26,856.5	26,199.6
Liabilities:
Total liabilities, fair value	299.7	203.9
Assets measured on recurring basis | Level 3
Assets:
Fixed maturities, including securities pledged, Fair Value	594.0	579.2
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	1.5	1.5
Assets held in separate accounts	0.7	2.4
Total assets, fair value	634.2	619.7
Liabilities:
Total liabilities, fair value	175.6	129.2
Reinsurance Agreements | Assets measured on recurring basis
Liabilities:
Derivatives	1.0	(13.0)
Reinsurance Agreements | Assets measured on recurring basis | Level 1
Liabilities:
Derivatives	0	0
Reinsurance Agreements | Assets measured on recurring basis | Level 2
Liabilities:
Derivatives	1.0	(13.0)
Reinsurance Agreements | Assets measured on recurring basis | Level 3
Liabilities:
Derivatives	0	0
Interest rate contracts | Assets measured on recurring basis
Assets:
Derivatives	625.3	537.2
Liabilities:
Derivatives	287.8	209.2
Interest rate contracts | Assets measured on recurring basis | Level 1
Assets:
Derivatives	0	0
Liabilities:
Derivatives	0	0
Interest rate contracts | Assets measured on recurring basis | Level 2
Assets:
Derivatives	625.3	537.2
Liabilities:
Derivatives	287.8	209.2
Interest rate contracts | Assets measured on recurring basis | Level 3
Assets:
Derivatives	0	0
Liabilities:
Derivatives	0	0
Foreign exchange contracts | Assets measured on recurring basis
Assets:
Derivatives	26.0	18.3
Liabilities:
Derivatives	10.9	7.7
Foreign exchange contracts | Assets measured on recurring basis | Level 1
Assets:
Derivatives	0	0
Liabilities:
Derivatives	0	0
Foreign exchange contracts | Assets measured on recurring basis | Level 2
Assets:
Derivatives	26.0	18.3
Liabilities:
Derivatives	10.9	7.7
Foreign exchange contracts | Assets measured on recurring basis | Level 3
Assets:
Derivatives	0	0
Liabilities:
Derivatives	0	0
Equity contract | Assets measured on recurring basis
Liabilities:
Derivatives	0.1	0.1
Equity contract | Assets measured on recurring basis | Level 1
Liabilities:
Derivatives	0.1	0.1
Equity contract | Assets measured on recurring basis | Level 2
Liabilities:
Derivatives	0	0
Equity contract | Assets measured on recurring basis | Level 3
Liabilities:
Derivatives	0	0
Credit contracts | Assets measured on recurring basis
Assets:
Derivatives	6.0	6.5
Credit contracts | Assets measured on recurring basis | Level 1
Assets:
Derivatives	0	0
Credit contracts | Assets measured on recurring basis | Level 2
Assets:
Derivatives	6.0	6.5
Credit contracts | Assets measured on recurring basis | Level 3
Assets:
Derivatives	0	0
Stabilizer and MCGs | Assets measured on recurring basis
Liabilities:
Product guarantees	148.1	102.9
Stabilizer and MCGs | Assets measured on recurring basis | Level 1
Liabilities:
Product guarantees	0	0
Stabilizer and MCGs | Assets measured on recurring basis | Level 2
Liabilities:
Product guarantees	0	0
Stabilizer and MCGs | Assets measured on recurring basis | Level 3
Liabilities:
Product guarantees	148.1	102.9
FIA | Assets measured on recurring basis
Liabilities:
Product guarantees	27.5	26.3
FIA | Assets measured on recurring basis | Level 1
Liabilities:
Product guarantees	0	0
FIA | Assets measured on recurring basis | Level 2
Liabilities:
Product guarantees	0	0
FIA | Assets measured on recurring basis | Level 3
Liabilities:
Product guarantees	27.5	26.3
Fixed maturities
Assets:
Fixed maturities, including securities pledged, Fair Value	22,222.1	21,616.6
U.S. Treasuries
Assets:
Fixed maturities, including securities pledged, Fair Value	810.6	773.1
U.S. Treasuries | Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged, Fair Value	810.6	773.1
U.S. Treasuries | Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged, Fair Value	748.8	712.9
U.S. Treasuries | Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged, Fair Value	61.8	60.2
U.S. Treasuries | Assets measured on recurring basis | Level 3
Assets:
Fixed maturities, including securities pledged, Fair Value	0	0
U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged, Fair Value	46.2	46.6
U.S. government agencies and authorities | Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged, Fair Value	46.2	46.6
U.S. government agencies and authorities | Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged, Fair Value	0	0
U.S. government agencies and authorities | Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged, Fair Value	46.2	46.6
U.S. government agencies and authorities | Assets measured on recurring basis | Level 3
Assets:
Fixed maturities, including securities pledged, Fair Value	0	0
State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged, Fair Value	325.1	277.2
State, municipalities and political subdivisions | Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged, Fair Value	325.1	277.2
State, municipalities and political subdivisions | Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged, Fair Value	0	0
State, municipalities and political subdivisions | Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged, Fair Value	325.1	277.2
State, municipalities and political subdivisions | Assets measured on recurring basis | Level 3
Assets:
Fixed maturities, including securities pledged, Fair Value	0	0
U.S. corporate securities
Assets:
Fixed maturities, including securities pledged, Fair Value	11,650.9	11,220.0
U.S. corporate securities | Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged, Fair Value	11,650.9	11,220.0
U.S. corporate securities | Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged, Fair Value	0	0
U.S. corporate securities | Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged, Fair Value	11,264.4	10,845.2
U.S. corporate securities | Assets measured on recurring basis | Level 3
Assets:
Fixed maturities, including securities pledged, Fair Value	386.5	374.8
Foreign
Assets:
Fixed maturities, including securities pledged, Fair Value	363.8	364.8
Foreign | Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged, Fair Value	5,841.8	5,777.9
Foreign | Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged, Fair Value	0	0
Foreign | Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged, Fair Value	5,659.4	5,612.2
Foreign | Assets measured on recurring basis | Level 3
Assets:
Fixed maturities, including securities pledged, Fair Value	182.4	165.7
Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	2,051.4	2,043.4
Residential mortgage-backed | Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged, Fair Value	2,051.4	2,043.4
Residential mortgage-backed | Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged, Fair Value	0	0
Residential mortgage-backed | Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged, Fair Value	2,028.4	2,026.1
Residential mortgage-backed | Assets measured on recurring basis | Level 3
Assets:
Fixed maturities, including securities pledged, Fair Value	23.0	17.3
Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	1,146.2	1,078.0
Commercial mortgage-backed | Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged, Fair Value	1,146.2	1,078.0
Commercial mortgage-backed | Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged, Fair Value	0	0
Commercial mortgage-backed | Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged, Fair Value	1,146.2	1,059.0
Commercial mortgage-backed | Assets measured on recurring basis | Level 3
Assets:
Fixed maturities, including securities pledged, Fair Value	0	19.0
Other asset-backed Securities | Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged, Fair Value	349.9	400.4
Other asset-backed Securities | Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged, Fair Value	0	0
Other asset-backed Securities | Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged, Fair Value	347.8	398.0
Other asset-backed Securities | Assets measured on recurring basis | Level 3
Assets:
Fixed maturities, including securities pledged, Fair Value	2.1	2.4
Equity securities
Assets:
Equity securities, available-for-sale	123.6	121.9
Equity securities | Assets measured on recurring basis
Assets:
Equity securities, available-for-sale	123.6	121.9
Equity securities | Assets measured on recurring basis | Level 1
Assets:
Equity securities, available-for-sale	85.6	85.3
Equity securities | Assets measured on recurring basis | Level 2
Assets:
Equity securities, available-for-sale	0	0
Equity securities | Assets measured on recurring basis | Level 3
Assets:
Equity securities, available-for-sale	$ 38.0	$ 36.6