Fair Value Measurements - Fair Value Measurement (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Assets:
Equity securities, available-for-sale	$ 83.9	$ 131.3
Derivatives	658.1	450.3
Liabilities:
Derivatives	261.5	115.1
Fixed maturities
Assets:
Fixed maturities, including securities pledged, Fair Value	24,530.9	22,258.8
U.S. Treasuries
Assets:
Fixed maturities, including securities pledged, Fair Value	818.2	721.4
U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged, Fair Value	4.2	4.3
State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged, Fair Value	766.3	595.8
U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged, Fair Value	10,251.0	9,600.5
U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged, Fair Value	2,583.7	2,359.9
Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged, Fair Value	2,886.2	2,812.2
Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged, Fair Value	2,874.8	2,711.4
Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	2,667.8	1,929.2
Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	1,400.9	1,274.9
Equity securities
Assets:
Equity securities, available-for-sale	83.9	131.3
Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged, Fair Value	24,530.9	22,258.8
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	1,160.6	902.6
Assets held in separate accounts	60,028.8	58,910.6
Total assets, fair value	86,462.3	82,653.6
Liabilities:
Total liabilities, fair value	555.5	227.9
Assets measured on recurring basis | Reinsurance agreements
Liabilities:
Embedded derivative on reinsurance	0.8	(71.6)
Assets measured on recurring basis | Stabilizer and MCGs
Liabilities:
Product guarantees	272.0	161.3
Assets measured on recurring basis | FIA
Liabilities:
Product guarantees	21.2	23.1
Assets measured on recurring basis | Interest rate contracts
Assets:
Derivatives	637.8	422.4
Liabilities:
Derivatives	251.7	104.0
Assets measured on recurring basis | Foreign exchange contracts
Assets:
Derivatives	15.6	24.6
Liabilities:
Derivatives	9.5	10.7
Assets measured on recurring basis | Equity contract
Assets:
Derivatives	2.1	0.0
Liabilities:
Derivatives	0.0	0.1
Assets measured on recurring basis | Credit contracts
Assets:
Derivatives	2.6	3.3
Liabilities:
Derivatives	0.3	0.3
Assets measured on recurring basis | U.S. Treasuries
Assets:
Fixed maturities, including securities pledged, Fair Value	818.2	721.4
Assets measured on recurring basis | U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged, Fair Value	4.2	4.3
Assets measured on recurring basis | State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged, Fair Value	766.3	595.8
Assets measured on recurring basis | U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged, Fair Value	10,251.0	9,600.5
Assets measured on recurring basis | U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged, Fair Value	2,583.7	2,359.9
Assets measured on recurring basis | Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged, Fair Value	2,886.2	2,812.2
Assets measured on recurring basis | Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged, Fair Value	2,874.8	2,711.4
Assets measured on recurring basis | Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	2,667.8	1,929.2
Assets measured on recurring basis | Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	1,400.9	1,274.9
Assets measured on recurring basis | Other asset-backed Securities
Assets:
Fixed maturities, including securities pledged, Fair Value	277.8	249.2
Assets measured on recurring basis | Equity securities
Assets:
Equity securities, available-for-sale	83.9	131.3
Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged, Fair Value	751.7	660.4
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	1,160.6	902.6
Assets held in separate accounts	55,210.0	54,283.0
Total assets, fair value	57,157.5	55,929.8
Liabilities:
Total liabilities, fair value	0.3	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	0.1	0.0
Liabilities:
Derivatives	0.3	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, Fair Value	751.7	660.4
Assets measured on recurring basis | Level 1 | U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 1 | State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 1 | U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 1 | U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 1 | Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 1 | Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 1 | Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 1 | Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 1 | Other asset-backed Securities
Assets:
Fixed maturities, including securities pledged, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 1 | Equity securities
Assets:
Equity securities, available-for-sale	35.1	83.8
Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged, Fair Value	23,095.0	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	4,815.4	4,623.6
Total assets, fair value	28,568.4	26,061.1
Liabilities:
Total liabilities, fair value	262.0	42.8
Assets measured on recurring basis | Level 2 | Reinsurance agreements
Liabilities:
Embedded derivative on reinsurance	0.8	(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	637.7	422.4
Liabilities:
Derivatives	251.4	103.4
Assets measured on recurring basis | Level 2 | Foreign exchange contracts
Assets:
Derivatives	15.6	24.6
Liabilities:
Derivatives	9.5	10.7
Assets measured on recurring basis | Level 2 | Equity contract
Assets:
Derivatives	2.1	0.0
Liabilities:
Derivatives	0.0	0.0
Assets measured on recurring basis | Level 2 | Credit contracts
Assets:
Derivatives	2.6	3.3
Liabilities:
Derivatives	0.3	0.3
Assets measured on recurring basis | Level 2 | U.S. Treasuries
Assets:
Fixed maturities, including securities pledged, Fair Value	66.5	61.0
Assets measured on recurring basis | Level 2 | U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged, Fair Value	4.2	4.3
Assets measured on recurring basis | Level 2 | State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged, Fair Value	766.3	595.8
Assets measured on recurring basis | Level 2 | U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged, Fair Value	10,230.3	9,598.2
Assets measured on recurring basis | Level 2 | U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged, Fair Value	2,162.2	1,963.5
Assets measured on recurring basis | Level 2 | Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged, Fair Value	2,886.1	2,811.7
Assets measured on recurring basis | Level 2 | Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged, Fair Value	2,706.9	2,553.3
Assets measured on recurring basis | Level 2 | Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	2,642.4	1,901.0
Assets measured on recurring basis | Level 2 | Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	1,391.7	1,262.3
Assets measured on recurring basis | Level 2 | Other asset-backed Securities
Assets:
Fixed maturities, including securities pledged, Fair Value	238.4	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, Fair Value	684.2	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	3.4	4.0
Total assets, fair value	736.4	662.7
Liabilities:
Total liabilities, fair value	293.2	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	272.0	161.3
Assets measured on recurring basis | Level 3 | FIA
Liabilities:
Product guarantees	21.2	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, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 3 | U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 3 | State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged, Fair Value	0.0	0.0
Assets measured on recurring basis | Level 3 | U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged, Fair Value	20.7	2.3
Assets measured on recurring basis | Level 3 | U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged, Fair Value	421.5	396.4
Assets measured on recurring basis | Level 3 | Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged, Fair Value	0.1	0.5
Assets measured on recurring basis | Level 3 | Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged, Fair Value	167.9	158.1
Assets measured on recurring basis | Level 3 | Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	25.4	28.2
Assets measured on recurring basis | Level 3 | Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged, Fair Value	9.2	12.6
Assets measured on recurring basis | Level 3 | Other asset-backed Securities
Assets:
Fixed maturities, including securities pledged, Fair Value	39.4	13.1
Assets measured on recurring basis | Level 3 | Equity securities
Assets:
Equity securities, available-for-sale	$ 48.8	$ 47.5