Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Financial Assets
Beginning balance	$ 371.4	$ 355.1
Transfers into Level 3	72.2	27.2
Transfers out of Level 3	(8.0)	(33.2)
Total gains or losses
Net investment losses included in net income related to financial assets	(1.0)	0.0
Net unrealized investment gains (losses) included in OCI	(13.9)	(3.2)
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(49.5)	(5.7)
Ending balance	371.2	340.2
Financial Liabilities
Beginning balance	106.6	104.5
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	(5.2)	4.3
Net unrealized investment gains (losses) included in OCI	0.0	0.0
Purchases	0.0	0.0
Issuances	0.9	0.7
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(3.2)	(1.9)
Ending balance	99.1	107.6
Municipal bonds
Financial Assets
Beginning balance	60.8	59.6
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	(3.2)	0.0
Total gains or losses
Net investment losses included in net income related to financial assets	0.0	0.0
Net unrealized investment gains (losses) included in OCI	(3.4)	(0.9)
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	54.1	58.6
Corporate bonds
Financial Assets
Beginning balance	210.3	155.8
Transfers into Level 3	67.5	24.1
Transfers out of Level 3	0.0	(27.3)
Total gains or losses
Net investment losses included in net income related to financial assets	0.0	0.0
Net unrealized investment gains (losses) included in OCI	(6.4)	(2.0)
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(45.4)	(1.5)
Ending balance	226.0	149.1
Mortgage and Asset Backed Securities
Financial Assets
Beginning balance	98.9	139.4
Transfers into Level 3	4.7	3.1
Transfers out of Level 3	(4.8)	(5.9)
Total gains or losses
Net investment losses included in net income related to financial assets	(0.9)	0.0
Net unrealized investment gains (losses) included in OCI	(4.1)	(0.3)
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(4.0)	(4.1)
Ending balance	89.8	132.2
Total
Financial Assets
Beginning balance	370.0	354.8
Transfers into Level 3	72.2	27.2
Transfers out of Level 3	(8.0)	(33.2)
Total gains or losses
Net investment losses included in net income related to financial assets	(0.9)	0.0
Net unrealized investment gains (losses) included in OCI	(13.9)	(3.2)
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(49.5)	(5.7)
Ending balance	369.9	339.9
Equity securities
Financial Assets
Beginning balance	1.4	0.3
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Total gains or losses
Net investment losses included in net income related to financial assets	(0.1)	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	$ 1.3	$ 0.3

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