Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financial Assets
Beginning balance	$ 324.9	$ 399.7	$ 355.1	$ 295.2
Transfers into Level 3	59.7	21.7	118.5	215.7
Transfers out of Level 3	0.0	(100.2)	(75.9)	(166.7)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(6.5)	(0.3)	(7.6)	(0.3)
Net unrealized investment gains (losses) included in OCI	6.2	10.0	8.8	(10.5)
Purchases	0.0	0.0	0.0	8.8
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	(22.2)	(3.2)	(36.8)	(14.5)
Ending balance	362.1	327.7	362.1	327.7
Financial Liabilities
Beginning balance	108.9	93.6	104.5	93.7
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	0.7	4.4	8.2	5.3
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	1.4	2.0	3.3	5.9
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(4.3)	(1.9)	(9.3)	(6.8)
Ending balance	106.7	98.1	106.7	98.1
Municipal bonds
Financial Assets
Beginning balance	58.6	73.2	59.6	44.3
Transfers into Level 3	0.0	6.2	0.0	80.7
Transfers out of Level 3	0.0	(16.7)	0.0	(62.6)
Total gains or losses
Net investment gains (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	(0.3)	3.2	(0.9)	4.0
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)	(0.2)	(0.5)	(0.7)
Ending balance	58.2	65.7	58.2	65.7
Corporate bonds
Financial Assets
Beginning balance	150.5	126.3	155.8	104.0
Transfers into Level 3	55.7	6.8	108.3	39.6
Transfers out of Level 3	0.0	(12.5)	(56.7)	(26.7)
Total gains or losses
Net investment gains (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	(0.1)	0.5	1.0	0.3
Purchases	0.0	0.0	0.0	6.9
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	(1.1)	0.1	(3.4)	(2.9)
Ending balance	205.0	121.2	205.0	121.2
Mortgage and Asset Backed Securities
Financial Assets
Beginning balance	115.5	200.1	139.4	146.8
Transfers into Level 3	4.0	8.7	10.2	95.4
Transfers out of Level 3	0.0	(71.0)	(19.2)	(77.4)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(6.6)	(0.3)	(7.7)	(0.3)
Net unrealized investment gains (losses) included in OCI	6.6	6.3	8.7	(14.8)
Purchases	0.0	0.0	0.0	1.9
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	(21.0)	(3.1)	(32.9)	(10.9)
Ending balance	98.5	140.7	98.5	140.7
Total Fixed Maturity Securities
Financial Assets
Beginning balance	324.6	399.6	354.8	295.1
Transfers into Level 3	59.7	21.7	118.5	215.7
Transfers out of Level 3	0.0	(100.2)	(75.9)	(166.7)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(6.6)	(0.3)	(7.7)	(0.3)
Net unrealized investment gains (losses) included in OCI	6.2	10.0	8.8	(10.5)
Purchases	0.0	0.0	0.0	8.8
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	(22.2)	(3.2)	(36.8)	(14.5)
Ending balance	361.7	327.6	361.7	327.6
Equity securities
Financial Assets
Beginning balance	0.3	0.1	0.3	0.1
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 assets	0.1	0.0	0.1	0.0
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	0.0	0.0	0.0	0.0
Ending balance	$ 0.4	$ 0.1	$ 0.4	$ 0.1

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