Fair Value Measurements - Fair Value Measurement (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Assets:
Equity securities, available-for-sale	$ 84.4	$ 131.3
Derivatives	594.9	450.3
Assets held in separate accounts	61,647.2	58,910.6
Liabilities:
Derivatives	235.7	115.1
Fixed maturities
Assets:
Fixed maturities, including securities pledged	25,513.3	22,258.8
U.S. Treasuries
Assets:
Fixed maturities, including securities pledged	786.4	721.4
U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged	4.1	4.3
State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged	823.5	595.8
U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged	10,416.2	9,600.5
U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged	2,833.0	2,359.9
Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged	2,992.3	2,812.2
Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged	2,942.8	2,711.4
Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged	2,906.2	1,929.2
Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged	1,458.7	1,274.9
Equity securities
Assets:
Equity securities, available-for-sale	84.4	131.3
Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged	25,513.3	22,258.8
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	848.3	902.6
Assets held in separate accounts	61,647.2	58,910.6
Total assets, fair value	88,688.1	82,653.6
Liabilities:
Total liabilities, fair value	526.1	227.9
Assets measured on recurring basis | Reinsurance agreements
Liabilities:
Embedded derivative on reinsurance	1.5	(71.6)
Assets measured on recurring basis | Stabilizer and MCGs
Liabilities:
Product guarantees	267.1	161.3
Assets measured on recurring basis | FIA
Liabilities:
Product guarantees	21.8	23.1
Assets measured on recurring basis | Interest rate contracts
Assets:
Derivatives	574.6	422.4
Liabilities:
Derivatives	226.1	104.0
Assets measured on recurring basis | Foreign exchange contracts
Assets:
Derivatives	14.0	24.6
Liabilities:
Derivatives	9.3	10.7
Assets measured on recurring basis | Equity contract
Assets:
Derivatives	2.3	0.0
Liabilities:
Derivatives	0.0	0.1
Assets measured on recurring basis | Credit contracts
Assets:
Derivatives	4.0	3.3
Liabilities:
Derivatives	0.3	0.3
Assets measured on recurring basis | U.S. Treasuries
Assets:
Fixed maturities, including securities pledged	786.4	721.4
Assets measured on recurring basis | U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged	4.1	4.3
Assets measured on recurring basis | State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged	823.5	595.8
Assets measured on recurring basis | U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged	10,416.2	9,600.5
Assets measured on recurring basis | U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged	2,833.0	2,359.9
Assets measured on recurring basis | Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged	2,992.3	2,812.2
Assets measured on recurring basis | Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged	2,942.8	2,711.4
Assets measured on recurring basis | Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged	2,906.2	1,929.2
Assets measured on recurring basis | Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged	1,458.7	1,274.9
Assets measured on recurring basis | Other asset-backed Securities
Assets:
Fixed maturities, including securities pledged	350.1	249.2
Assets measured on recurring basis | Equity securities
Assets:
Equity securities, available-for-sale	84.4	131.3
Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged	715.0	660.4
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	848.3	902.6
Assets held in separate accounts	56,877.8	54,283.0
Total assets, fair value	58,476.8	55,929.8
Liabilities:
Total liabilities, fair value	0.0	0.7
Assets measured on recurring basis | Level 1 | Reinsurance agreements
Liabilities:
Embedded derivative on reinsurance	0.0	0.0
Assets measured on recurring basis | Level 1 | Stabilizer and MCGs
Liabilities:
Product guarantees	0.0	0.0
Assets measured on recurring basis | Level 1 | FIA
Liabilities:
Product guarantees	0.0	0.0
Assets measured on recurring basis | Level 1 | Interest rate contracts
Assets:
Derivatives	0.7	0.0
Liabilities:
Derivatives	0.0	0.6
Assets measured on recurring basis | Level 1 | Foreign exchange contracts
Assets:
Derivatives	0.0	0.0
Liabilities:
Derivatives	0.0	0.0
Assets measured on recurring basis | Level 1 | Equity contract
Assets:
Derivatives	0.0	0.0
Liabilities:
Derivatives	0.0	0.1
Assets measured on recurring basis | Level 1 | Credit contracts
Assets:
Derivatives	0.0	0.0
Liabilities:
Derivatives	0.0	0.0
Assets measured on recurring basis | Level 1 | U.S. Treasuries
Assets:
Fixed maturities, including securities pledged	715.0	660.4
Assets measured on recurring basis | Level 1 | U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 1 | State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 1 | U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 1 | U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 1 | Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 1 | Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 1 | Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 1 | Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 1 | Other asset-backed Securities
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 1 | Equity securities
Assets:
Equity securities, available-for-sale	35.0	83.8
Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged	24,023.6	20,987.2
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	0.0	0.0
Assets held in separate accounts	4,760.3	4,623.6
Total assets, fair value	29,378.1	26,061.1
Liabilities:
Total liabilities, fair value	237.2	42.8
Assets measured on recurring basis | Level 2 | Reinsurance agreements
Liabilities:
Embedded derivative on reinsurance	1.5	(71.6)
Assets measured on recurring basis | Level 2 | Stabilizer and MCGs
Liabilities:
Product guarantees	0.0	0.0
Assets measured on recurring basis | Level 2 | FIA
Liabilities:
Product guarantees	0.0	0.0
Assets measured on recurring basis | Level 2 | Interest rate contracts
Assets:
Derivatives	573.9	422.4
Liabilities:
Derivatives	226.1	103.4
Assets measured on recurring basis | Level 2 | Foreign exchange contracts
Assets:
Derivatives	14.0	24.6
Liabilities:
Derivatives	9.3	10.7
Assets measured on recurring basis | Level 2 | Equity contract
Assets:
Derivatives	2.3	0.0
Liabilities:
Derivatives	0.0	0.0
Assets measured on recurring basis | Level 2 | Credit contracts
Assets:
Derivatives	4.0	3.3
Liabilities:
Derivatives	0.3	0.3
Assets measured on recurring basis | Level 2 | U.S. Treasuries
Assets:
Fixed maturities, including securities pledged	71.4	61.0
Assets measured on recurring basis | Level 2 | U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged	4.1	4.3
Assets measured on recurring basis | Level 2 | State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged	823.5	595.8
Assets measured on recurring basis | Level 2 | U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged	10,405.9	9,598.2
Assets measured on recurring basis | Level 2 | U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged	2,360.6	1,963.5
Assets measured on recurring basis | Level 2 | Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged	2,992.2	2,811.7
Assets measured on recurring basis | Level 2 | Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged	2,778.5	2,553.3
Assets measured on recurring basis | Level 2 | Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged	2,877.5	1,901.0
Assets measured on recurring basis | Level 2 | Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged	1,439.1	1,262.3
Assets measured on recurring basis | Level 2 | Other asset-backed Securities
Assets:
Fixed maturities, including securities pledged	270.8	236.1
Assets measured on recurring basis | Level 2 | Equity securities
Assets:
Equity securities, available-for-sale	0.0	0.0
Assets measured on recurring basis | Level 3
Assets:
Fixed maturities, including securities pledged	774.7	611.2
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	0.0	0.0
Assets held in separate accounts	9.1	4.0
Total assets, fair value	833.2	662.7
Liabilities:
Total liabilities, fair value	288.9	184.4
Assets measured on recurring basis | Level 3 | Reinsurance agreements
Liabilities:
Embedded derivative on reinsurance	0.0	0.0
Assets measured on recurring basis | Level 3 | Stabilizer and MCGs
Liabilities:
Product guarantees	267.1	161.3
Assets measured on recurring basis | Level 3 | FIA
Liabilities:
Product guarantees	21.8	23.1
Assets measured on recurring basis | Level 3 | Interest rate contracts
Assets:
Derivatives	0.0	0.0
Liabilities:
Derivatives	0.0	0.0
Assets measured on recurring basis | Level 3 | Foreign exchange contracts
Assets:
Derivatives	0.0	0.0
Liabilities:
Derivatives	0.0	0.0
Assets measured on recurring basis | Level 3 | Equity contract
Assets:
Derivatives	0.0	0.0
Liabilities:
Derivatives	0.0	0.0
Assets measured on recurring basis | Level 3 | Credit contracts
Assets:
Derivatives	0.0	0.0
Liabilities:
Derivatives	0.0	0.0
Assets measured on recurring basis | Level 3 | U.S. Treasuries
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 3 | U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 3 | State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged	0.0	0.0
Assets measured on recurring basis | Level 3 | U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged	10.3	2.3
Assets measured on recurring basis | Level 3 | U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged	472.4	396.4
Assets measured on recurring basis | Level 3 | Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged	0.1	0.5
Assets measured on recurring basis | Level 3 | Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged	164.3	158.1
Assets measured on recurring basis | Level 3 | Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged	28.7	28.2
Assets measured on recurring basis | Level 3 | Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged	19.6	12.6
Assets measured on recurring basis | Level 3 | Other asset-backed Securities
Assets:
Fixed maturities, including securities pledged	79.3	13.1
Assets measured on recurring basis | Level 3 | Equity securities
Assets:
Equity securities, available-for-sale	$ 49.4	$ 47.5