Fair Value Measurements - Level 3 Financial Instruments (Details) - Level 3 - Assets measured on recurring basis - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ 12.3	$ 1.2	$ 5.4	$ 4.0
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	0.0	0.0	0.0	0.0
Purchases	1.1	0.0	6.1	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	(2.5)	0.0	(2.7)	0.0
Settlements	0.0	0.0	0.0	0.0
Transfers in to Level 3	0.0	3.4	2.1	3.4
Transfers out of Level 3	(7.8)	(1.2)	(7.8)	(4.0)
Fair Value, assets, ending balance	3.1	3.4	3.1	3.4
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	(131.1)	(226.0)	(150.5)	(161.3)
Total Realized/Unrealized Gains (Losses) Included in Net Income	(9.1)	(44.8)	11.6	(108.4)
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
Issues	(1.0)	(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	(141.2)	(272.0)	(141.2)	(272.0)
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	(23.1)	(21.7)	(23.3)	(23.1)
Total Realized/Unrealized Gains (Losses) Included in Net Income	(0.2)	(0.3)	(0.6)	0.5
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)	0.1	(0.2)	0.2
Sales	0.0	0.0	0.0	0.0
Settlements	0.4	0.7	1.1	1.2
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	(23.0)	(21.2)	(23.0)	(21.2)
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	30.4	1.7	7.5	2.3
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	(0.1)	0.0	(0.1)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.5	0.4	0.4	0.9
Purchases	6.0	0.0	10.9	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	(0.7)	0.0	(0.7)
Settlements	(0.3)	0.0	(0.9)	(0.5)
Transfers in to Level 3	0.0	19.4	8.6	18.8
Transfers out of Level 3	(10.1)	0.0	0.0	0.0
Fair Value, assets, ending balance	26.5	20.7	26.5	20.7
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	564.9	393.5	524.7	396.4
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	(0.1)	0.0	(0.1)
Total Realized/Unrealized Gains (Losses) Included in OCI	3.5	5.9	4.3	14.2
Purchases	7.9	37.0	44.6	43.1
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	(17.5)
Settlements	(0.4)	(20.8)	(0.9)	(52.2)
Transfers in to Level 3	28.8	6.0	32.0	44.2
Transfers out of Level 3	(9.6)	0.0	(9.6)	(6.6)
Fair Value, assets, ending balance	595.1	421.5	595.1	421.5
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	0.5	0.1	0.5
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	(0.4)	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	(0.4)	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	161.1	179.6	154.0	158.1
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.1	(1.7)	0.1	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.3	5.9	(1.1)	6.9
Purchases	0.0	0.0	18.1	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	(14.6)	0.0	(0.1)
Settlements	0.0	(4.1)	(9.9)	(13.9)
Transfers in to Level 3	0.0	2.8	0.0	19.6
Transfers out of Level 3	(33.1)	0.0	(32.8)	(2.7)
Fair Value, assets, ending balance	128.4	167.9	128.4	167.9
Change in Unrealized Gains (Losses) Included in Earnings	0.1	(0.1)	0.1	0.0
Residential mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	30.4	26.9	21.2	28.2
Total Realized/Unrealized Gains (Losses) Included in Net Income	(2.1)	(1.5)	(3.7)	(2.8)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.2	0.0	(0.1)	0.0
Purchases	17.4	0.0	17.4	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	0.8	0.0	2.0	0.0
Transfers out of Level 3	(9.9)	0.0	0.0	0.0
Fair Value, assets, ending balance	36.8	25.4	36.8	25.4
Change in Unrealized Gains (Losses) Included in Earnings	(2.1)	(1.5)	(3.7)	(2.8)
Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	6.0	0.1	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	0.1
Purchases	2.0	0.0	2.0	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(1.8)	(1.1)	(3.0)	(2.5)
Transfers in to Level 3	0.0	10.2	0.0	0.0
Transfers out of Level 3	0.0	(0.1)	(2.6)	(1.0)
Fair Value, assets, ending balance	6.2	9.2	6.2	9.2
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	22.5	12.6	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	0.1	0.2	0.5	0.0
Purchases	24.4	26.9	24.4	26.9
Issuances	0.0	0.0	0.0	0.0
Sales	(0.3)	0.0	0.0	0.0
Settlements	(0.3)	(0.3)	(0.7)	(0.6)
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	(10.5)	0.0	(15.0)	0.0
Fair Value, assets, ending balance	35.9	39.4	35.9	39.4
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	815.4	614.9	744.0	611.2
Total Realized/Unrealized Gains (Losses) Included in Net Income	(2.0)	(3.8)	(3.6)	(3.4)
Total Realized/Unrealized Gains (Losses) Included in OCI	4.6	12.5	4.0	22.1
Purchases	57.7	63.9	117.4	70.0
Issuances	0.0	0.0	0.0	0.0
Sales	(0.3)	(15.3)	0.0	(18.3)
Settlements	(2.8)	(26.3)	(15.4)	(69.7)
Transfers in to Level 3	29.6	38.4	42.6	82.6
Transfers out of Level 3	(73.2)	(0.1)	(60.0)	(10.3)
Fair Value, assets, ending balance	829.0	684.2	829.0	684.2
Change in Unrealized Gains (Losses) Included in Earnings	(2.0)	(2.0)	(3.6)	(3.3)
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	49.9	48.2	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	0.4	0.6	0.7	1.3
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	50.3	48.8	50.3	48.8
Change in Unrealized Gains (Losses) Included in Earnings	$ 0.0	$ 0.0	$ 0.0	$ 0.0