Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Financial Assets
Beginning balance	$ 355.1	$ 295.2
Transfers into Level 3	27.2	105.1
Transfers out of Level 3	(33.2)	(13.0)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	0.0	0.0
Net unrealized investment gains (losses) included in OCI	(3.2)	(32.9)
Purchases	0.0	8.8
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(5.7)	(6.0)
Ending balance	340.2	357.2
Financial Liabilities
Beginning balance	104.5	93.7
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Total gains or losses
Net investment (gains) losses included in net income related to financial liabilities	4.3	(5.0)
Net unrealized investment gains (losses) included in OCI	0.0	0.0
Purchases	0.0	0.0
Issuances	0.7	1.3
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(1.9)	(2.5)
Ending balance	107.6	87.5
Municipal bonds
Financial Assets
Beginning balance	59.6	44.3
Transfers into Level 3	0.0	63.7
Transfers out of Level 3	0.0	0.0
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	0.0	0.0
Net unrealized investment gains (losses) included in OCI	(0.9)	(3.0)
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(0.1)	(0.1)
Ending balance	58.6	104.9
Corporate bonds
Financial Assets
Beginning balance	155.8	104.0
Transfers into Level 3	24.1	18.9
Transfers out of Level 3	(27.3)	(10.1)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	0.0	0.0
Net unrealized investment gains (losses) included in OCI	(2.0)	(5.5)
Purchases	0.0	6.9
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(1.5)	(2.5)
Ending balance	149.1	111.7
Mortgage and Asset Backed Securities
Financial Assets
Beginning balance	139.4	146.8
Transfers into Level 3	3.1	22.5
Transfers out of Level 3	(5.9)	(2.9)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	0.0	0.0
Net unrealized investment gains (losses) included in OCI	(0.3)	(24.4)
Purchases	0.0	1.9
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(4.1)	(3.4)
Ending balance	132.2	140.5
Total Fixed Maturity Securities
Financial Assets
Beginning balance	354.8	295.1
Transfers into Level 3	27.2	105.1
Transfers out of Level 3	(33.2)	(13.0)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	0.0	0.0
Net unrealized investment gains (losses) included in OCI	(3.2)	(32.9)
Purchases	0.0	8.8
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(5.7)	(6.0)
Ending balance	339.9	357.1
Equity securities
Financial Assets
Beginning balance	0.3	0.1
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	0.0	0.0
Net unrealized investment 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
Paydowns, maturities and distributions	0.0	0.0
Ending balance	$ 0.3	$ 0.1

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