Fair Value Measurements - Level 3 Financial Instruments (Details) - Assets measured on recurring basis - Level 3 - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Separate Accounts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	$ 3.4	$ 0.0	$ 4.0	$ 2.4
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.2	0.0	0.2	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0	0.0	0.0
Purchases	4.1	4.0	4.1	4.0
Issuances	0.0	0.0	0.0	0.0
Sales	(0.7)	0.0	(0.7)	0.0
Settlements	0.0	0.0	0.0	0.0
Transfers in to Level 3	2.1	0.0	5.5	0.0
Transfers out of Level 3	0.0	0.0	(4.0)	(2.4)
Fair Value, assets, ending balance	9.1	4.0	9.1	4.0
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, assets, beginning balance	20.7	12.1	2.3	19.3
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	(0.1)	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	(0.1)	0.6	(0.1)
Purchases	0.0	30.7	0.0	30.7
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	(0.6)	0.0
Settlements	0.0	0.0	(0.5)	(0.8)
Transfers in to Level 3	0.0	0.0	8.6	0.0
Transfers out of Level 3	(10.4)	(9.6)	0.0	(16.0)
Fair Value, assets, ending balance	10.3	33.1	10.3	33.1
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, assets, beginning balance	421.5	369.4	396.4	355.5
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	(0.1)	(0.1)	(0.1)
Total Realized/Unrealized Gains (Losses) Included in OCI	5.3	(0.6)	19.4	(6.0)
Purchases	59.9	5.7	103.0	63.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	(17.5)	0.0
Settlements	(14.3)	(2.7)	(66.4)	(56.8)
Transfers in to Level 3	0.0	0.0	44.2	0.0
Transfers out of Level 3	0.0	(16.1)	(6.6)	0.0
Fair Value, assets, ending balance	472.4	355.6	472.4	355.6
Change in Unrealized Gains (Losses) Included in Earnings	0.0	(0.1)	(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, assets, beginning balance	0.1	1.7	0.5	0.0
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	(0.4)	(1.7)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0	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	(1.2)	0.0	(2.1)
Transfers in to Level 3	0.0	0.0	0.0	4.4
Transfers out of Level 3	0.0	0.0	0.0	0.0
Fair Value, assets, ending balance	0.1	0.5	0.1	0.5
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	(0.4)	(1.7)
Foreign corporate private securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	167.9	167.0	158.1	165.7
Total Realized/Unrealized Gains (Losses) Included in Net Income	(0.6)	0.0	(0.6)	0.2
Total Realized/Unrealized Gains (Losses) Included in OCI	1.6	0.5	8.6	0.2
Purchases	0.0	0.0	0.0	1.8
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	(0.1)	0.0
Settlements	(4.6)	(9.9)	(18.5)	(27.2)
Transfers in to Level 3	0.0	0.0	19.6	16.1
Transfers out of Level 3	0.0	(0.8)	(2.8)	0.0
Fair Value, assets, ending balance	164.3	156.8	164.3	156.8
Change in Unrealized Gains (Losses) Included in Earnings	(0.6)	0.0	(0.6)	0.0
Residential mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	25.4	21.5	28.2	17.3
Total Realized/Unrealized Gains (Losses) Included in Net Income	(1.8)	(1.1)	(4.6)	(2.6)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.1	0.2	0.1	(0.7)
Purchases	5.0	0.0	5.0	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, assets, ending balance	28.7	20.6	28.7	20.6
Change in Unrealized Gains (Losses) Included in Earnings	(1.8)	(1.1)	(4.6)	(2.6)
Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	9.2	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	0.2	0.0
Purchases	11.3	10.0	11.3	10.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(1.0)	(1.0)	(3.5)	(1.0)
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	(1.0)	(19.0)
Fair Value, assets, ending balance	19.6	9.0	19.6	9.0
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, assets, beginning balance	39.4	13.8	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	0.0	0.0	0.0	0.0
Purchases	66.6	0.0	67.2	12.4
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(0.3)	(0.4)	(1.0)	(0.5)
Transfers in to Level 3	0.0	0.0	0.0	5.3
Transfers out of Level 3	(26.4)	0.0	0.0	(6.2)
Fair Value, assets, ending balance	79.3	13.4	79.3	13.4
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	684.2	585.5	611.2	579.2
Total Realized/Unrealized Gains (Losses) Included in Net Income	(2.4)	(1.2)	(5.8)	(4.2)
Total Realized/Unrealized Gains (Losses) Included in OCI	7.1	0.0	28.9	(6.7)
Purchases	142.8	46.4	186.5	117.9
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	(18.2)	0.0
Settlements	(20.2)	(15.2)	(89.9)	(88.4)
Transfers in to Level 3	0.0	0.0	72.4	32.4
Transfers out of Level 3	(36.8)	(26.5)	(10.4)	(41.2)
Fair Value, assets, ending balance	774.7	589.0	774.7	589.0
Change in Unrealized Gains (Losses) Included in Earnings	(2.4)	(1.2)	(5.7)	(4.4)
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, assets, beginning balance	48.8	37.0	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	0.2	1.9	0.6
Purchases	0.0	10.3	0.0	10.3
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	49.4	47.5	49.4	47.5
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, assets, 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	(1.5)	0.0	(1.5)
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	0.0	0.0	0.0	0.0
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	(272.0)	(92.0)	(161.3)	(102.9)
Total Realized/Unrealized Gains (Losses) Included in Net Income	6.1	(69.5)	(102.3)	(56.3)
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	(1.2)	(1.1)	(3.5)	(3.4)
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	(267.1)	(162.6)	(267.1)	(162.6)
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	(21.2)	(27.2)	(23.1)	(26.3)
Total Realized/Unrealized Gains (Losses) Included in Net Income	(1.6)	3.8	(1.1)	2.6
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)	0.1	(0.1)
Sales	0.0	0.0	0.0	0.0
Settlements	1.1	0.9	2.3	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	(21.8)	(22.6)	(21.8)	(22.6)
Change In Unrealized Gains (Losses) Included in Earnings	$ 0.0	$ 0.0	$ 0.0	$ 0.0