Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Financial Assets
Beginning balance	$ 425.3	$ 371.4
Transfers into Level 3	6.3	72.2
Transfers out of Level 3	0.0	(8.0)
Total gains or losses
Net investment losses included in net income related to financial assets	0.0	(1.0)
Net unrealized investment gains (losses) included in OCI	1.7	(13.9)
Purchases	19.2	0.0
Issuances	0.0	0.0
Sales	(2.6)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(3.9)	(49.5)
Ending balance	446.0	371.2
Financial Liabilities
Beginning balance	91.0	106.6
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	0.2	5.2
Net unrealized investment gains (losses) included in OCI	0.0	0.0
Purchases	0.0	0.0
Issuances	2.1	0.9
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(3.4)	(3.2)
Ending balance	89.9	99.1
Municipal bonds
Financial Assets
Beginning balance	54.4	60.8
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	(3.2)
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	1.2	(3.4)
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(0.2)	(0.1)
Ending balance	55.4	54.1
Corporate bonds
Financial Assets
Beginning balance	261.3	210.3
Transfers into Level 3	5.9	67.5
Transfers out of Level 3	0.0	0.0
Total gains or losses
Net investment losses included in net income related to financial assets	0.0	0.0
Net unrealized investment gains (losses) included in OCI	0.9	(6.4)
Purchases	19.0	0.0
Issuances	0.0	0.0
Sales	(2.6)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(0.5)	(45.4)
Ending balance	284.0	226.0
Mortgage Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	107.6	98.9
Transfers into Level 3	0.4	4.7
Transfers out of Level 3	0.0	(4.8)
Total gains or losses
Net investment losses included in net income related to financial assets	0.0	(0.9)
Net unrealized investment gains (losses) included in OCI	(0.4)	(4.1)
Purchases	0.2	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(3.2)	(4.0)
Ending balance	104.6	89.8
Total Fixed Maturity Securities
Financial Assets
Beginning balance	423.3	370.0
Transfers into Level 3	6.3	72.2
Transfers out of Level 3	0.0	(8.0)
Total gains or losses
Net investment losses included in net income related to financial assets	0.0	(0.9)
Net unrealized investment gains (losses) included in OCI	1.7	(13.9)
Purchases	19.2	0.0
Issuances	0.0	0.0
Sales	(2.6)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(3.9)	(49.5)
Ending balance	444.0	369.9
Equity securities
Financial Assets
Beginning balance	2.0	1.4
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	0.0	(0.1)
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	$ 2.0	$ 1.3

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