Fair Value Measurements - Fair Value Measurement (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Assets:
Equity securities, available-for-sale	$ 85.2	$ 82.3
Derivatives	168.2	470.8
Assets held in separate accounts	68,373.3	61,980.7
Liabilities:
Derivatives	51.9	172.9
Fixed maturities
Assets:
Fixed maturities, including securities pledged	24,798.9	24,895.0
U.S. Treasuries
Assets:
Fixed maturities, including securities pledged	715.7	787.0
U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged	3.6	3.8
State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged	858.0	795.4
U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged	9,539.2	9,972.4
U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged	3,295.2	2,960.6
Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged	2,816.8	2,884.4
Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged	3,013.4	2,910.0
Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged	2,700.1	2,788.0
Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged	1,302.3	1,317.7
Other asset-backed securities
Assets:
Fixed maturities, including securities pledged	554.6	475.7
Equity securities
Assets:
Equity securities, available-for-sale	85.2	82.3
Assets measured on recurring basis
Assets:
Fixed maturities, including securities pledged	24,798.9	24,895.0
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	725.9	972.7
Assets held in separate accounts	68,373.3	61,980.7
Total assets, fair value	94,151.5	88,401.5
Liabilities:
Total liabilities, fair value	188.5	303.2
Assets measured on recurring basis | FIA
Liabilities:
Product guarantees	23.0	23.3
Assets measured on recurring basis | Stabilizer and MCGs
Liabilities:
Product guarantees	141.2	150.5
Assets measured on recurring basis | Credit contracts
Assets:
Derivatives	8.8	5.6
Liabilities:
Derivatives	6.4	0.2
Assets measured on recurring basis | Interest rate contracts
Assets:
Derivatives	148.7	437.5
Liabilities:
Derivatives	24.7	154.4
Assets measured on recurring basis | Foreign exchange contracts
Assets:
Derivatives	6.8	25.4
Liabilities:
Derivatives	19.3	18.3
Assets measured on recurring basis | Equity contract
Assets:
Derivatives	3.9	2.3
Liabilities:
Derivatives	1.5	0.0
Assets measured on recurring basis | Embedded derivative on reinsurance
Liabilities:
Embedded derivative on reinsurance	(27.6)	(43.5)
Assets measured on recurring basis | U.S. Treasuries
Assets:
Fixed maturities, including securities pledged	715.7	787.0
Assets measured on recurring basis | U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged	3.6	3.8
Assets measured on recurring basis | State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged	858.0	795.4
Assets measured on recurring basis | U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged	9,539.2	9,972.4
Assets measured on recurring basis | U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged	3,295.2	2,960.6
Assets measured on recurring basis | Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged	2,816.8	2,884.4
Assets measured on recurring basis | Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged	3,013.4	2,910.0
Assets measured on recurring basis | Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged	2,700.1	2,788.0
Assets measured on recurring basis | Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged	1,302.3	1,317.7
Assets measured on recurring basis | Other asset-backed securities
Assets:
Fixed maturities, including securities pledged	554.6	475.7
Assets measured on recurring basis | Equity securities
Assets:
Equity securities, available-for-sale	85.2	82.3
Assets measured on recurring basis | Level 1
Assets:
Fixed maturities, including securities pledged	656.0	729.3
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	721.6	939.8
Assets held in separate accounts	63,804.5	57,192.4
Total assets, fair value	65,217.6	58,895.9
Liabilities:
Total liabilities, fair value	0.0	0.6
Assets measured on recurring basis | Level 1 | FIA
Liabilities:
Product guarantees	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 | Credit contracts
Assets:
Derivatives	0.0	0.0
Liabilities:
Derivatives	0.0	0.0
Assets measured on recurring basis | Level 1 | Interest rate contracts
Assets:
Derivatives	0.6	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	0.0
Assets measured on recurring basis | Level 1 | Embedded derivative on reinsurance
Liabilities:
Embedded derivative on reinsurance	0.0	0.0
Assets measured on recurring basis | Level 1 | U.S. Treasuries
Assets:
Fixed maturities, including securities pledged	656.0	729.3
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	34.9	34.4
Assets measured on recurring basis | Level 2
Assets:
Fixed maturities, including securities pledged	23,313.9	23,421.7
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	4.3	32.9
Assets held in separate accounts	4,565.7	4,782.9
Total assets, fair value	28,051.5	28,708.3
Liabilities:
Total liabilities, fair value	24.3	128.8
Assets measured on recurring basis | Level 2 | FIA
Liabilities:
Product guarantees	0.0	0.0
Assets measured on recurring basis | Level 2 | Stabilizer and MCGs
Liabilities:
Product guarantees	0.0	0.0
Assets measured on recurring basis | Level 2 | Credit contracts
Assets:
Derivatives	8.8	5.6
Liabilities:
Derivatives	6.4	0.2
Assets measured on recurring basis | Level 2 | Interest rate contracts
Assets:
Derivatives	148.1	437.5
Liabilities:
Derivatives	24.7	153.8
Assets measured on recurring basis | Level 2 | Foreign exchange contracts
Assets:
Derivatives	6.8	25.4
Liabilities:
Derivatives	19.3	18.3
Assets measured on recurring basis | Level 2 | Equity contract
Assets:
Derivatives	3.9	2.3
Liabilities:
Derivatives	1.5	0.0
Assets measured on recurring basis | Level 2 | Embedded derivative on reinsurance
Liabilities:
Embedded derivative on reinsurance	(27.6)	(43.5)
Assets measured on recurring basis | Level 2 | U.S. Treasuries
Assets:
Fixed maturities, including securities pledged	59.7	57.7
Assets measured on recurring basis | Level 2 | U.S. government agencies and authorities
Assets:
Fixed maturities, including securities pledged	3.6	3.8
Assets measured on recurring basis | Level 2 | State, municipalities and political subdivisions
Assets:
Fixed maturities, including securities pledged	858.0	795.4
Assets measured on recurring basis | Level 2 | U.S. corporate public securities
Assets:
Fixed maturities, including securities pledged	9,512.7	9,964.9
Assets measured on recurring basis | Level 2 | U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged	2,700.1	2,435.9
Assets measured on recurring basis | Level 2 | Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged	2,816.7	2,884.3
Assets measured on recurring basis | Level 2 | Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged	2,885.0	2,756.0
Assets measured on recurring basis | Level 2 | Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged	2,663.3	2,766.8
Assets measured on recurring basis | Level 2 | Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged	1,296.1	1,307.9
Assets measured on recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturities, including securities pledged	518.7	449.0
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	829.0	744.0
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	0.0	0.0
Assets held in separate accounts	3.1	5.4
Total assets, fair value	882.4	797.3
Liabilities:
Total liabilities, fair value	164.2	173.8
Assets measured on recurring basis | Level 3 | FIA
Liabilities:
Product guarantees	23.0	23.3
Assets measured on recurring basis | Level 3 | Stabilizer and MCGs
Liabilities:
Product guarantees	141.2	150.5
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 | 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 | Embedded derivative on reinsurance
Liabilities:
Embedded derivative on reinsurance	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	26.5	7.5
Assets measured on recurring basis | Level 3 | U.S. corporate private securities
Assets:
Fixed maturities, including securities pledged	595.1	524.7
Assets measured on recurring basis | Level 3 | Foreign corporate public securities and foreign governments
Assets:
Fixed maturities, including securities pledged	0.1	0.1
Assets measured on recurring basis | Level 3 | Foreign corporate private securities
Assets:
Fixed maturities, including securities pledged	128.4	154.0
Assets measured on recurring basis | Level 3 | Residential mortgage-backed
Assets:
Fixed maturities, including securities pledged	36.8	21.2
Assets measured on recurring basis | Level 3 | Commercial mortgage-backed
Assets:
Fixed maturities, including securities pledged	6.2	9.8
Assets measured on recurring basis | Level 3 | Other asset-backed securities
Assets:
Fixed maturities, including securities pledged	35.9	26.7
Assets measured on recurring basis | Level 3 | Equity securities
Assets:
Equity securities, available-for-sale	$ 50.3	$ 47.9