Fair Value Measurements - Level 3 Financial Instruments (Details) - Assets measured on recurring basis - Level 3 - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Separate Accounts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Equity securities, available-for-sale, beginning balance	$ 1.2	$ 0.7	$ 4.0	$ 2.4
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Transfers in to Level 3	3.4	0.0	3.4	0.0
Transfers out of Level 3	(1.2)	(0.7)	(4.0)	(2.4)
Fair Value, Equity securities, available-for-sale, ending balance	3.4	0.0	3.4	0.0
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	0.0
U.S. corporate public securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Fixed maturities, including securities pledged, beginning balance	1.7	18.3	2.3	19.3
Total Realized/Unrealized Gains (Losses) Included in Net Income	(0.1)	0.0	(0.1)	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.4	0.0	0.9	(0.1)
Purchases	0.0	0.0	0.0	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	(0.7)	0.0	(0.7)	0.0
Settlements	0.0	0.0	(0.5)	(0.8)
Transfers in to Level 3	19.4	0.0	18.8	0.0
Transfers out of Level 3	0.0	(6.2)	0.0	(6.3)
Fair Value, Fixed maturities, including securities pledged, ending balance	20.7	12.1	20.7	12.1
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	0.0
U.S. corporate private securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Fixed maturities, including securities pledged, beginning balance	393.5	368.2	396.4	355.5
Total Realized/Unrealized Gains (Losses) Included in Net Income	(0.1)	0.0	(0.1)	(0.1)
Total Realized/Unrealized Gains (Losses) Included in OCI	5.9	(7.8)	14.2	(5.3)
Purchases	37.0	26.3	43.1	73.4
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	(17.5)	0.0
Settlements	(20.8)	(17.3)	(52.2)	(54.1)
Transfers in to Level 3	6.0	0.0	44.2	0.0
Transfers out of Level 3	0.0	0.0	(6.6)	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	421.5	369.4	421.5	369.4
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	(0.1)	(0.1)
Foreign corporate public securities and foreign governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Fixed maturities, including securities pledged, beginning balance	0.5	1.2	0.5	0.0
Total Realized/Unrealized Gains (Losses) Included in Net Income	(0.4)	(1.7)	(0.4)	(1.7)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.1	0.0	(0.1)
Purchases	0.0	0.0	0.0	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	(0.2)	0.0	(0.9)
Transfers in to Level 3	0.0	3.6	0.0	4.4
Transfers out of Level 3	0.0	(1.3)	0.0	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	0.1	1.7	0.1	1.7
Change in Unrealized Gains (Losses) Included in Earnings	(0.4)	(1.7)	(0.4)	(1.7)
Foreign corporate private securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Fixed maturities, including securities pledged, beginning balance	179.6	181.2	158.1	165.7
Total Realized/Unrealized Gains (Losses) Included in Net Income	(1.7)	(0.5)	0.0	(0.3)
Total Realized/Unrealized Gains (Losses) Included in OCI	5.9	(1.9)	6.9	(0.4)
Purchases	0.0	0.0	0.0	1.7
Issuances	0.0	0.0	0.0	0.0
Sales	(14.6)	0.0	(0.1)	0.0
Settlements	(4.1)	(15.1)	(13.9)	(19.1)
Transfers in to Level 3	2.8	3.3	19.6	19.4
Transfers out of Level 3	0.0	0.0	(2.7)	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	167.9	167.0	167.9	167.0
Change in Unrealized Gains (Losses) Included in Earnings	(0.1)	(0.5)	0.0	(0.5)
Residential mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Fixed maturities, including securities pledged, beginning balance	26.9	23.0	28.2	17.3
Total Realized/Unrealized Gains (Losses) Included in Net Income	(1.5)	(0.9)	(2.8)	(1.5)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	(0.6)	0.0	(0.9)
Purchases	0.0	0.0	0.0	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Transfers in to Level 3	0.0	0.0	0.0	6.6
Transfers out of Level 3	0.0	0.0	0.0	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	25.4	21.5	25.4	21.5
Change in Unrealized Gains (Losses) Included in Earnings	(1.5)	(0.9)	(2.8)	(1.5)
Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Fixed maturities, including securities pledged, beginning balance	0.1	0.0	12.6	19.0
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.1	0.0	0.1	0.0
Purchases	0.0	0.0	0.0	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(1.1)	0.0	(2.5)	0.0
Transfers in to Level 3	10.2	0.0	0.0	0.0
Transfers out of Level 3	(0.1)	0.0	(1.0)	(19.0)
Fair Value, Fixed maturities, including securities pledged, ending balance	9.2	0.0	9.2	0.0
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	0.0
Other asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Fixed maturities, including securities pledged, beginning balance	12.6	2.1	13.1	2.4
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.2	0.0	0.0	(0.1)
Purchases	26.9	12.4	26.9	12.4
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(0.3)	(0.1)	(0.6)	(0.1)
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	(0.6)	0.0	(0.8)
Fair Value, Fixed maturities, including securities pledged, ending balance	39.4	13.8	39.4	13.8
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	0.0
Fixed maturities, available-for-sale, including securities pledged
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Fixed maturities, including securities pledged, beginning balance	614.9	594.0	611.2	579.2
Total Realized/Unrealized Gains (Losses) Included in Net Income	(3.8)	(3.1)	(3.4)	(3.6)
Total Realized/Unrealized Gains (Losses) Included in OCI	12.5	(10.2)	22.1	(6.9)
Purchases	63.9	38.7	70.0	87.5
Issuances	0.0	0.0	0.0	0.0
Sales	(15.3)	0.0	(18.3)	0.0
Settlements	(26.3)	(32.7)	(69.7)	(75.0)
Transfers in to Level 3	38.4	6.9	82.6	30.4
Transfers out of Level 3	(0.1)	(8.1)	(10.3)	(26.1)
Fair Value, Fixed maturities, including securities pledged, ending balance	684.2	585.5	684.2	585.5
Change in Unrealized Gains (Losses) Included in Earnings	(2.0)	(3.1)	(3.3)	(3.8)
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Equity securities, available-for-sale, beginning balance	48.2	38.0	47.5	36.6
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.6	(1.0)	1.3	0.4
Purchases	0.0	0.0	0.0	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Fair Value, Equity securities, available-for-sale, ending balance	48.8	37.0	48.8	37.0
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	0.0
Short-term investments and cash equivalents
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Fixed maturities, including securities pledged, beginning balance	0.0	1.5	0.0	1.5
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	0.0	1.5	0.0	1.5
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	0.0
Stabilizer (Investment Only) and MCG Contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair Value, Derivatives, beginning balance	(226.0)	(148.1)	(161.3)	(102.9)
Total Realized/Unrealized Gains (Losses) Included in Net Income	(44.8)	57.3	(108.4)	13.2
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Issues	(1.2)	(1.2)	(2.3)	(2.3)
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Fair Value, Derivatives, ending balance	(272.0)	(92.0)	(272.0)	(92.0)
Change In Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	0.0
FIA
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair Value, Derivatives, beginning balance	(21.7)	(27.5)	(23.1)	(26.3)
Total Realized/Unrealized Gains (Losses) Included in Net Income	(0.3)	0.2	0.5	(1.2)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Issues	0.1	0.0	0.2	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.7	0.1	1.2	0.3
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Fair Value, Derivatives, ending balance	(21.2)	(27.2)	(21.2)	(27.2)
Change In Unrealized Gains (Losses) Included in Earnings	$ 0.0	$ 0.0	$ 0.0	$ 0.0