Fair Value Measurements - Level 3 Financial Instruments (Details) - Level 3 - Assets measured on recurring basis - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Assets held in separate accounts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	$ 3.1	$ 3.4	$ 5.4	$ 4.0
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.1	0.2	0.1	0.2
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0	0.0	0.0
Purchases	3.8	4.1	9.9	4.1
Issuances	0.0	0.0	0.0	0.0
Sales	(0.3)	(0.7)	(3.0)	(0.7)
Settlements	0.0	0.0	0.0	0.0
Transfers in to Level 3	0.0	2.1	2.1	5.5
Transfers out of Level 3	(1.1)	0.0	(8.9)	(4.0)
Fair Value, assets, ending balance	5.6	9.1	5.6	9.1
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	(141.2)	(272.0)	(150.5)	(161.3)
Total Realized/Unrealized Gains (Losses) Included in Net Income	8.1	6.1	19.7	(102.3)
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	(0.8)	(1.2)	(3.1)	(3.5)
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	(133.9)	(267.1)	(133.9)	(267.1)
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	(23.0)	(21.2)	(23.3)	(23.1)
Total Realized/Unrealized Gains (Losses) Included in Net Income	(0.9)	(1.6)	(1.5)	(1.1)
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	0.2	(0.1)	0.0	0.1
Sales	0.0	0.0	0.0	0.0
Settlements	1.4	1.1	2.5	2.3
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	(22.3)	(21.8)	(22.3)	(21.8)
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, assets, beginning balance	26.5	20.7	7.5	2.3
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	0.0	(0.1)
Total Realized/Unrealized Gains (Losses) Included in OCI	(0.1)	0.0	0.4	0.6
Purchases	0.0	0.0	10.9	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	(0.6)
Settlements	0.0	0.0	(1.0)	(0.5)
Transfers in to Level 3	0.0	0.0	8.6	8.6
Transfers out of Level 3	0.0	(10.4)	0.0	0.0
Fair Value, assets, ending balance	26.4	10.3	26.4	10.3
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, assets, beginning balance	595.1	421.5	524.7	396.4
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	1.2	5.3	5.8	19.4
Purchases	1.8	59.9	46.4	103.0
Issuances	0.0	0.0	0.0	0.0
Sales	(1.1)	0.0	(1.1)	(17.5)
Settlements	(9.6)	(14.3)	(10.6)	(66.4)
Transfers in to Level 3	11.5	0.0	43.3	44.2
Transfers out of Level 3	0.0	0.0	(9.6)	(6.6)
Fair Value, assets, ending balance	599.0	472.4	599.0	472.4
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	(0.1)
Foreign corporate public securities and foreign governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	0.1	0.1	0.1	0.5
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	0.0	(0.4)
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, assets, ending balance	0.1	0.1	0.1	0.1
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	(0.4)
Foreign corporate private securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	128.4	167.9	154.0	158.1
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	(0.6)	0.1	(0.6)
Total Realized/Unrealized Gains (Losses) Included in OCI	(0.9)	1.6	(2.0)	8.6
Purchases	13.6	0.0	31.7	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	(0.1)
Settlements	(0.6)	(4.6)	(10.5)	(18.5)
Transfers in to Level 3	0.0	0.0	0.0	19.6
Transfers out of Level 3	0.0	0.0	(32.8)	(2.8)
Fair Value, assets, ending balance	140.5	164.3	140.5	164.3
Change in Unrealized Gains (Losses) Included in Earnings	0.0	(0.6)	0.1	(0.6)
Residential mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	36.8	25.4	21.2	28.2
Total Realized/Unrealized Gains (Losses) Included in Net Income	(1.6)	(1.8)	(5.2)	(4.6)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.1	0.1	(0.1)	0.1
Purchases	9.0	5.0	9.0	5.0
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	2.0	0.0
Transfers out of Level 3	(17.4)	0.0	0.0	0.0
Fair Value, assets, ending balance	26.9	28.7	26.9	28.7
Change in Unrealized Gains (Losses) Included in Earnings	(1.6)	(1.8)	(5.3)	(4.6)
Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	6.2	9.2	9.8	12.6
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	0.0	0.1	0.0	0.2
Purchases	7.0	11.3	8.0	11.3
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(0.2)	(1.0)	(0.2)	(3.5)
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	(5.2)	0.0	(9.8)	(1.0)
Fair Value, assets, ending balance	7.8	19.6	7.8	19.6
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, assets, beginning balance	35.9	39.4	26.7	13.1
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	0.7	0.0
Purchases	32.8	66.6	32.8	67.2
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(0.4)	(0.3)	(1.1)	(1.0)
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	(24.4)	(26.4)	(15.0)	0.0
Fair Value, assets, ending balance	44.1	79.3	44.1	79.3
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	0.0
Fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	829.0	684.2	744.0	611.2
Total Realized/Unrealized Gains (Losses) Included in Net Income	(1.5)	(2.4)	(5.0)	(5.8)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.5	7.1	4.8	28.9
Purchases	64.2	142.8	138.8	186.5
Issuances	0.0	0.0	0.0	0.0
Sales	(1.1)	0.0	(1.1)	(18.2)
Settlements	(10.8)	(20.2)	(23.4)	(89.9)
Transfers in to Level 3	11.5	0.0	53.9	72.4
Transfers out of Level 3	(47.0)	(36.8)	(67.2)	(10.4)
Fair Value, assets, ending balance	844.8	774.7	844.8	774.7
Change in Unrealized Gains (Losses) Included in Earnings	(1.6)	(2.4)	(5.2)	(5.7)
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	50.3	48.8	47.9	47.5
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	0.1	0.6	0.8	1.9
Purchases	0.0	0.0	1.7	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, assets, ending balance	50.4	49.4	50.4	49.4
Change in Unrealized Gains (Losses) Included in Earnings	$ 0.0	$ 0.0	$ 0.0	$ 0.0