Fair Value Measurements - Level 3 Financial Instruments (Details) - Assets measured on recurring basis - Level 3 - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 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	$ 0.7	$ 17.9	$ 2.4	$ 13.1
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	0.0	0.0	0.0	0.0
Purchases	0.0	0.1	0.0	5.9
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	(2.2)	0.0	(3.2)
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	(0.7)	0.0	(2.4)	0.0
Fair Value, Equity securities, available-for-sale, ending balance	0.0	15.9	0.0	15.9
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	2.5	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	(2.5)	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
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	1.2	0.0	0.0	0.0
Total Realized/Unrealized Gains (Losses) Included in Net Income	(1.7)	0.0	(1.7)	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	0.1	0.0	(0.1)	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	(0.2)	0.0	(0.9)	0.0
Transfers in to Level 3	(3.6)	0.0	(4.4)	0.0
Transfers out of Level 3	(1.3)	0.0	0.0	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	1.7	0.0	1.7	0.0
Change in Unrealized Gains (Losses) Included in Earnings	(1.7)	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	181.2	34.8	165.7	42.8
Total Realized/Unrealized Gains (Losses) Included in Net Income	(0.5)	0.0	(0.3)	0.0
Total Realized/Unrealized Gains (Losses) Included in OCI	(1.9)	(0.6)	(0.4)	1.9
Purchases	0.0	56.3	1.7	56.3
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(15.1)	0.0	(19.1)	0.0
Transfers in to Level 3	(3.3)	(68.4)	(19.4)	(66.8)
Transfers out of Level 3	0.0	(13.3)	0.0	(22.2)
Fair Value, Fixed maturities, including securities pledged, ending balance	167.0	145.6	167.0	145.6
Change in Unrealized Gains (Losses) Included in Earnings	(0.5)	0.0	(0.5)	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	18.3	43.3	19.3	39.3
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	0.0	0.2	(0.1)	0.9
Purchases	0.0	5.5	0.0	10.8
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	(0.9)	(0.8)	(2.9)
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	(6.2)	0.0	(6.3)	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	12.1	48.0	12.1	48.0
Change in Unrealized Gains (Losses) Included in Earnings	0.0	(0.1)	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	368.2	140.1	355.5	106.0
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	(7.8)	0.2	(5.3)	1.9
Purchases	26.3	37.6	73.4	70.5
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(17.3)	0.0	(54.1)	0.0
Transfers in to Level 3	0.0	(45.5)	0.0	(45.0)
Transfers out of Level 3	0.0	0.0	0.0	0.0
Fair Value, Fixed maturities, including securities pledged, ending balance	369.4	223.4	369.4	223.4
Change in Unrealized Gains (Losses) Included in Earnings	0.0	0.0	(0.1)	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	23.0	23.1	17.3	23.7
Total Realized/Unrealized Gains (Losses) Included in Net Income	(0.9)	(0.7)	(1.5)	(1.1)
Total Realized/Unrealized Gains (Losses) Included in OCI	(0.6)	0.1	(0.9)	0.1
Purchases	0.0	0.3	0.0	0.3
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	(6.6)	0.0
Transfers out of Level 3	0.0	(1.6)	0.0	(1.8)
Fair Value, Fixed maturities, including securities pledged, ending balance	21.5	21.2	21.5	21.2
Change in Unrealized Gains (Losses) Included in Earnings	(0.9)	(0.7)	(1.5)	(1.1)
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	14.0	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	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	(14.0)	(19.0)	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
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	2.1	11.1	2.4	17.7
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.0	0.0	0.0	0.7
Total Realized/Unrealized Gains (Losses) Included in OCI	0.0	0.0	(0.1)	(0.5)
Purchases	12.4	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	(0.1)	(0.2)	(0.1)	(7.1)
Transfers in to Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	(0.6)	(8.5)	(0.8)	(8.4)
Fair Value, Fixed maturities, including securities pledged, ending balance	13.8	2.4	13.8	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	594.0	268.9	579.2	234.6
Total Realized/Unrealized Gains (Losses) Included in Net Income	(3.1)	(0.8)	(3.6)	(0.5)
Total Realized/Unrealized Gains (Losses) Included in OCI	(10.2)	(0.1)	(6.9)	4.3
Purchases	38.7	99.7	87.5	137.9
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	(32.7)	(1.1)	(75.0)	(10.0)
Transfers in to Level 3	(6.9)	(113.9)	(30.4)	(111.8)
Transfers out of Level 3	(8.1)	(39.9)	(26.1)	(37.5)
Fair Value, Fixed maturities, including securities pledged, ending balance	585.5	440.6	585.5	440.6
Change in Unrealized Gains (Losses) Included in Earnings	(3.1)	(0.8)	(3.8)	(1.2)
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Equity securities, available-for-sale, beginning balance	38.0	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	(1.0)	0.0	0.4	1.3
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, Equity securities, available-for-sale, ending balance	37.0	37.2	37.0	37.2
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	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, Fixed maturities, including securities pledged, ending balance	1.5	0.0	1.5	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	(148.1)	(18.0)	(102.9)	0.0
Total Realized/Unrealized Gains (Losses) Included in Net Income	57.3	(12.8)	13.2	(29.6)
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	(1.2)	(1.2)	(2.3)	(2.4)
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	(92.0)	(32.0)	(92.0)	(32.0)
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	(27.5)	(24.3)	(26.3)	(23.1)
Total Realized/Unrealized Gains (Losses) Included in Net Income	0.2	(1.4)	(1.2)	(2.4)
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	0.0	0.0	0.0	(0.2)
Sales	0.0	0.0	0.0	0.0
Settlements	0.1	0.0	0.3	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	$ (27.2)	$ (25.7)	$ (27.2)	$ (25.7)