Fair Value of Financial Instruments - Transfers Between Different Fair Value Levels (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Financial Assets
Beginning balance	$ 355.1	$ 295.2
Transfers into Level 3	172.6	268.8
Transfers out of Level 3	(83.6)	(190.3)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	8.1	0.3
Net unrealized investment gains (losses) included in OCI	6.3	(4.8)
Purchases	0.0	8.8
Issuances	0.0	0.0
Sales	0.0	1.2
Settlements	0.0	0.0
Paydowns, maturities and distributions	(70.9)	(23.5)
Ending balance	371.4	355.1
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	10.0	11.5
Net unrealized investment gains (losses) included in OCI	0.0	0.0
Purchases	0.0	0.0
Issuances	4.9	8.4
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(12.8)	(9.1)
Ending balance	106.6	104.5
Total
Financial Assets
Beginning balance	354.8	295.1
Transfers into Level 3	171.6	268.6
Transfers out of Level 3	(83.6)	(190.3)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(8.2)	(0.3)
Net unrealized investment gains (losses) included in OCI	6.3	(4.8)
Purchases	0.0	8.8
Issuances	0.0	0.0
Sales	0.0	1.2
Settlements	0.0	0.0
Paydowns, maturities and distributions	(70.9)	(23.5)
Ending balance	370.0	354.8
Municipal bonds
Financial Assets
Beginning balance	59.6	44.3
Transfers into Level 3	18.6	80.7
Transfers out of Level 3	0.0	(69.1)
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	(2.5)	4.3
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(14.9)	(0.6)
Ending balance	60.8	59.6
Corporate bonds
Financial Assets
Beginning balance	155.8	104.0
Transfers into Level 3	131.7	83.6
Transfers out of Level 3	(64.4)	(36.6)
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	1.4
Purchases	0.0	6.9
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(12.8)	(3.5)
Ending balance	210.3	155.8
Mortgage-Backed and Other Asset -Backed Securities
Financial Assets
Beginning balance	139.4	146.8
Transfers into Level 3	21.3	104.3
Transfers out of Level 3	(19.2)	(84.6)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(8.2)	(0.3)
Net unrealized investment gains (losses) included in OCI	8.8	(10.5)
Purchases	0.0	1.9
Issuances	0.0	0.0
Sales	0.0	1.2
Settlements	0.0	0.0
Paydowns, maturities and distributions	(43.2)	(19.4)
Ending balance	98.9	139.4
Equity securities
Financial Assets
Beginning balance	0.3	0.1
Transfers into Level 3	1.0	0.2
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	0.1	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	$ 1.4	$ 0.3

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