Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Financial Assets
Beginning balance	$ 420.0	$ 324.9	$ 371.4	$ 355.1
Transfers into Level 3	33.5	59.7	179.3	118.5
Transfers out of Level 3	(34.8)	0.0	(42.9)	(75.9)
Total gains or losses
Net investment losses included in net income related to financial assets	(0.9)	(6.5)	(3.2)	(7.6)
Net unrealized investment gains (losses) included in OCI	(16.6)	6.2	(41.6)	8.8
Purchases	16.5	0.0	21.4	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	(2.1)	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(15.4)	(22.2)	(80.0)	(36.8)
Ending balance	402.3	362.1	402.3	362.1
Financial Liabilities
Beginning balance	93.2	108.9	106.6	104.5
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Total gains or losses
Net investment (gains) losses included in net income related to financial liabilities	(4.6)	(0.7)	(14.4)	(8.2)
Net unrealized investment gains (losses) included in OCI	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Issuances	2.7	1.4	4.8	3.3
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(1.4)	(4.3)	(7.1)	(9.3)
Ending balance	89.9	106.7	89.9	106.7
Municipal bonds
Financial Assets
Beginning balance	51.0	58.6	60.8	59.6
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	(3.2)	0.0
Total gains or losses
Net investment losses included in net income related to financial assets	0.0	0.0	0.0	0.0
Net unrealized investment gains (losses) included in OCI	(5.3)	(0.3)	(11.6)	(0.9)
Purchases	0.2	0.0	0.2	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
Paydowns, maturities and distributions	7.4	(0.1)	7.1	(0.5)
Ending balance	53.3	58.2	53.3	58.2
Corporate bonds
Financial Assets
Beginning balance	270.5	150.5	210.3	155.8
Transfers into Level 3	20.5	55.7	144.0	108.3
Transfers out of Level 3	(34.8)	0.0	(34.9)	(56.7)
Total gains or losses
Net investment losses included in net income related to financial assets	0.0	0.0	0.0	0.0
Net unrealized investment gains (losses) included in OCI	(7.4)	(0.1)	(18.7)	1.0
Purchases	8.4	0.0	13.3	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
Paydowns, maturities and distributions	(17.4)	(1.1)	(74.2)	(3.4)
Ending balance	239.8	205.0	239.8	205.0
Mortgage Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	97.1	115.5	98.9	139.4
Transfers into Level 3	12.2	4.0	34.5	10.2
Transfers out of Level 3	0.0	0.0	(4.8)	(19.2)
Total gains or losses
Net investment losses included in net income related to financial assets	(0.8)	(6.6)	(3.1)	(7.7)
Net unrealized investment gains (losses) included in OCI	(3.9)	6.6	(11.3)	8.7
Purchases	7.9	0.0	7.9	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	(2.1)	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(5.3)	(21.0)	(12.8)	(32.9)
Ending balance	107.2	98.5	107.2	98.5
Total Fixed Maturity Securities
Financial Assets
Beginning balance	418.6	324.6	370.0	354.8
Transfers into Level 3	32.7	59.7	178.5	118.5
Transfers out of Level 3	(34.8)	0.0	(42.9)	(75.9)
Total gains or losses
Net investment losses included in net income related to financial assets	(0.8)	(6.6)	(3.1)	(7.7)
Net unrealized investment gains (losses) included in OCI	(16.6)	6.2	(41.6)	8.8
Purchases	16.5	0.0	21.4	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	(2.1)	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(15.3)	(22.2)	(79.9)	(36.8)
Ending balance	400.3	361.7	400.3	361.7
Equity securities
Financial Assets
Beginning balance	1.4	0.3	1.4	0.3
Transfers into Level 3	0.8	0.0	0.8	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Total gains or losses
Net investment losses included in net income related to financial assets	(0.1)	0.1	(0.1)	0.1
Net unrealized investment 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
Paydowns, maturities and distributions	(0.1)	0.0	(0.1)	0.0
Ending balance	$ 2.0	$ 0.4	$ 2.0	$ 0.4

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