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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Separate Accounts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Equity securities, available-for-sale, beginning balance	$ 0.0	$ 15.9	$ 2.4	$ 13.1
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	0.0	0.0	0.0	0.0
Purchases	4.0	4.7	4.0	10.6
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	(1.3)	0.0	(4.5)
Settlements	0.0	0.0	0.0	0.0
Transfers in to Level 3	0.0	(0.9)	0.0	(0.9)
Transfers out of Level 3	0.0	(6.6)	(2.4)	(6.6)
Fair Value, Equity securities, available-for-sale, ending balance	4.0	13.6	4.0	13.6
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	0.0
U.S. government agencies and authorities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Fixed maturities, including securities pledged, beginning balance	0.0	0.0	0.0	5.1
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	0.0	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	(5.1)
Fair Value, Fixed maturities, including securities pledged, 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
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	12.1	48.0	19.3	39.3
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)	(0.6)	(0.1)	0.4
Purchases	30.7	6.9	30.7	13.2
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	(1.1)	(0.8)	(4.1)
Transfers in to Level 3	0.0	(1.0)	0.0	0.0
Transfers out of Level 3	(9.6)	(5.5)	(16.0)	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	33.1	48.7	33.1	48.7
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	(0.1)
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	369.4	223.4	355.5	106.0
Total Realized/Unrealized Gains (Losses) Included in Net Income	(0.1)	(0.1)	(0.1)	(0.1)
Total Realized/Unrealized Gains (Losses) Included in OCI	(0.6)	(9.3)	(6.0)	(1.5)
Purchases	5.7	10.8	63.0	94.5
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(2.7)	0.0	(56.8)	0.0
Transfers in to Level 3	0.0	(124.3)	0.0	(139.2)
Transfers out of Level 3	(16.1)	(11.0)	0.0	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	355.6	338.1	355.6	338.1
Change in Unrealized Gains (Losses) Included in Earnings	(0.1)	(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, Fixed maturities, including securities pledged, beginning balance	1.7	0.0	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	(1.7)	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0	(0.1)	0.0
Purchases	0.0	0.0	0.0	$ 0.0
Issuances	0.0	0.0	0.0
Sales	0.0	0.0	0.0	$ 0.0
Settlements	(1.2)	0.0	(2.1)	0.0
Transfers in to Level 3	0.0	0.0	(4.4)	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	0.5	0.0	0.5	0.0
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	(1.7)	0.0
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	167.0	145.6	165.7	42.8
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	0.2	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.5	(0.7)	0.2	1.2
Purchases	0.0	0.0	1.8	$ 56.3
Issuances	0.0	0.0	0.0
Sales	0.0	0.0	0.0	$ 0.0
Settlements	(9.9)	0.0	(27.2)	0.0
Transfers in to Level 3	0.0	0.0	(16.1)	(66.8)
Transfers out of Level 3	(0.8)	0.0	0.0	(22.2)
Fair Value, Fixed maturities, including securities pledged, ending balance	156.8	144.9	156.8	144.9
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	0.0	0.0
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	21.5	21.2	17.3	23.7
Total Realized/Unrealized Gains (Losses) Included in Net Income	(1.1)	(0.9)	(2.6)	(1.6)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.2	0.3	(0.7)	0.4
Purchases	0.0	3.3	0.0	3.5
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	(1.7)	(6.6)	0.0
Transfers out of Level 3	0.0	(6.6)	0.0	(7.0)
Fair Value, Fixed maturities, including securities pledged, ending balance	20.6	19.0	20.6	19.0
Change in Unrealized Gains (Losses) Included in Earnings	(1.1)	(0.9)	(2.6)	(1.6)
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	0.0	0.0	19.0	0.0
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	10.0	18.9	10.0	18.9
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(1.0)	0.0	(1.0)	0.0
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	(19.0)	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	9.0	18.9	9.0	18.9
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	13.8	2.4	2.4	17.7
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	0.0	0.8
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0	0.0	(0.6)
Purchases	0.0	0.0	12.4	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(0.4)	0.0	(0.5)	(8.1)
Transfers in to Level 3	0.0	0.0	(5.3)	0.0
Transfers out of Level 3	0.0	0.0	(6.2)	(7.4)
Fair Value, Fixed maturities, including securities pledged, ending balance	13.4	2.4	13.4	2.4
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	585.5	440.6	579.2	234.6
Total Realized/Unrealized Gains (Losses) Included in Net Income	(1.2)	(1.0)	(4.2)	(1.0)
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	(10.3)	(6.7)	(0.1)
Purchases	46.4	39.9	117.9	186.4
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(15.2)	(1.1)	(88.4)	(12.2)
Transfers in to Level 3	0.0	(127.0)	(32.4)	(206.0)
Transfers out of Level 3	(26.5)	(23.1)	(41.2)	(41.7)
Fair Value, Fixed maturities, including securities pledged, ending balance	589.0	572.0	589.0	572.0
Change in Unrealized Gains (Losses) Included in Earnings	(1.2)	(1.0)	(4.4)	(1.8)
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Equity securities, available-for-sale, beginning balance	37.0	37.2	36.6	35.9
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	0.2	0.6	1.5
Purchases	10.3	0.0	10.3	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	47.5	37.4	47.5	37.4
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	1.5	0.0	1.5	0.0
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	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	(1.5)	0.0	(1.5)	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	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, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs, Change in Unrealized Gain (Loss)	0.0	0.0	0.0	0.0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair Value, Derivatives, beginning balance	(92.0)	(32.0)	(102.9)	0.0
Total Realized/Unrealized Gains (Losses) Included in Net Income	(69.5)	(11.9)	(56.3)	(41.5)
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	(1.1)	(1.1)	(3.4)	(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	(162.6)	(45.0)	(162.6)	(45.0)
FIA
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs, Change in Unrealized Gain (Loss)	0.0	0.0	0.0	0.0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair Value, Derivatives, beginning balance	(27.2)	(25.7)	(26.3)	(23.1)
Total Realized/Unrealized Gains (Losses) Included in Net Income	3.8	(0.6)	2.6	(3.0)
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
Issues	(0.1)	0.0	(0.1)	(0.2)
Sales	0.0	0.0	0.0	0.0
Settlements	0.9	0.0	1.2	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	$ (22.6)	$ (26.3)	$ (22.6)	$ (26.3)