Fair Value Measurements - Level 3 Financial Instruments (Details) - Assets measured on recurring basis - Level 3 - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Separate Accounts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	$ 5.4	$ 4.0
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0
Purchases	5.0	0.0
Issuances	0.0	0.0
Sales	(0.2)	0.0
Settlements	0.0	0.0
Transfers in to Level 3	2.1	0.0
Transfers out of Level 3	0.0	(2.8)
Fair Value, assets, ending balance	12.3	1.2
Change in Unrealized Gains (Losses) Included in Earnings	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	7.5	2.3
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	(0.3)	(0.1)
Purchases	4.9	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	(0.7)	(0.5)
Transfers in to Level 3	19.0	0.0
Transfers out of Level 3	0.0	0.0
Fair Value, assets, ending balance	30.4	1.7
Change in Unrealized Gains (Losses) Included in Earnings	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	524.7	396.4
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	(0.1)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.1	8.3
Purchases	36.7	0.2
Issuances	0.0	0.0
Sales	0.0	(17.5)
Settlements	(0.5)	(31.4)
Transfers in to Level 3	3.9	44.2
Transfers out of Level 3	0.0	(6.6)
Fair Value, assets, ending balance	564.9	393.5
Change in Unrealized Gains (Losses) Included in Earnings	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	0.5
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Transfers in to Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Fair Value, assets, ending balance	0.1	0.5
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0
Foreign corporate private securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	154.0	158.1
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.1	0.1
Total Realized/Unrealized Gains (Losses) Included in OCI	(1.2)	4.3
Purchases	18.1	0.0
Issuances	0.0	0.0
Sales	0.0	(0.1)
Settlements	(9.9)	(7.7)
Transfers in to Level 3	0.0	29.7
Transfers out of Level 3	0.0	(4.8)
Fair Value, assets, ending balance	161.1	179.6
Change in Unrealized Gains (Losses) Included in Earnings	0.1	0.1
Residential mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	21.2	28.2
Total Realized/Unrealized Gains (Losses) Included in Net Income	(1.4)	(1.3)
Total Realized/Unrealized Gains (Losses) Included in OCI	(0.4)	0.0
Purchases	9.9	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Transfers in to Level 3	1.1	0.0
Transfers out of Level 3	0.0	0.0
Fair Value, assets, ending balance	30.4	26.9
Change in Unrealized Gains (Losses) Included in Earnings	(1.4)	(1.3)
Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	9.8	12.6
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0
Purchases	0.0	0.1
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	(1.2)	0.0
Transfers in to Level 3	0.0	0.0
Transfers out of Level 3	(2.6)	(12.6)
Fair Value, assets, ending balance	6.0	0.1
Change in Unrealized Gains (Losses) Included in Earnings	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	26.7	13.1
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.1	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.4	(0.2)
Purchases	10.4	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	(0.6)	(0.3)
Transfers in to Level 3	0.3	0.0
Transfers out of Level 3	(14.8)	0.0
Fair Value, assets, ending balance	22.5	12.6
Change in Unrealized Gains (Losses) Included in Earnings	0.1	0.0
Fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	744.0	611.2
Total Realized/Unrealized Gains (Losses) Included in Net Income	(1.2)	(1.3)
Total Realized/Unrealized Gains (Losses) Included in OCI	(1.4)	12.3
Purchases	80.0	0.3
Issuances	0.0	0.0
Sales	0.0	(17.6)
Settlements	(12.9)	(39.9)
Transfers in to Level 3	24.3	73.9
Transfers out of Level 3	(17.4)	(24.0)
Fair Value, assets, ending balance	815.4	614.9
Change in Unrealized Gains (Losses) Included in Earnings	(1.2)	(1.3)
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	47.9	47.5
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.3	0.7
Purchases	1.7	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Transfers in to Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Fair Value, assets, ending balance	49.9	48.2
Change in Unrealized Gains (Losses) Included in Earnings	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	(150.5)	(161.3)
Total Realized/Unrealized Gains (Losses) Included in Net Income	20.7	(63.6)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0
Purchases	0.0	0.0
Issues	(1.3)	(1.1)
Sales	0.0	0.0
Settlements	0.0	0.0
Transfers in to Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Fair Value, Derivatives, ending balance	(131.1)	(226.0)
Change In Unrealized Gains (Losses) Included in Earnings	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	(23.3)	(23.1)
Total Realized/Unrealized Gains (Losses) Included in Net Income	(0.4)	0.8
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0
Purchases	0.0	0.0
Issues	(0.1)	(0.1)
Sales	0.0	0.0
Settlements	0.7	0.5
Transfers in to Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Fair Value, Derivatives, ending balance	(23.1)	(21.9)
Change In Unrealized Gains (Losses) Included in Earnings	$ 0.0	$ 0.0