Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Financial Assets
Beginning balance	$ 340.2	$ 357.2	$ 355.1	$ 295.2
Transfers into Level 3	31.6	88.9	58.8	194.0
Transfers out of Level 3	(42.7)	(53.5)	(75.9)	(66.5)
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	4.8	12.4	1.6	(20.5)
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	(9.0)	(5.3)	(14.7)	(11.3)
Ending balance	324.9	399.7	324.9	399.7
Financial Liabilities
Beginning balance	107.6	87.5	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	3.2	6.0	7.5	0.9
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	1.2	2.5	1.9	3.9
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(3.1)	(2.4)	(5.0)	(4.9)
Ending balance	108.9	93.6	108.9	93.6
Municipal bonds
Financial Assets
Beginning balance	58.6	104.9	59.6	44.3
Transfers into Level 3	0.0	10.7	0.0	74.5
Transfers out of Level 3	0.0	(45.9)	0.0	(45.9)
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	3.8	(0.6)	0.8
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	(0.3)	(0.3)	(0.4)	(0.5)
Ending balance	58.6	73.2	58.6	73.2
Corporate bonds
Financial Assets
Beginning balance	149.1	111.7	155.8	104.0
Transfers into Level 3	28.5	14.0	52.6	32.8
Transfers out of Level 3	(29.4)	(4.2)	(56.7)	(14.2)
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	3.2	5.3	1.2	(0.2)
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	(0.9)	(0.5)	(2.4)	(3.0)
Ending balance	150.5	126.3	150.5	126.3
Mortgage and Asset Backed Securities
Financial Assets
Beginning balance	132.2	140.5	139.4	146.8
Transfers into Level 3	3.1	64.2	6.2	86.7
Transfers out of Level 3	(13.3)	(3.4)	(19.2)	(6.4)
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	1.3	3.3	1.0	(21.1)
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	(7.8)	(4.5)	(11.9)	(7.8)
Ending balance	115.5	200.1	115.5	200.1
Total Fixed Maturity Securities
Financial Assets
Beginning balance	339.9	357.1	354.8	295.1
Transfers into Level 3	31.6	88.9	58.8	194.0
Transfers out of Level 3	(42.7)	(53.5)	(75.9)	(66.5)
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	4.8	12.4	1.6	(20.5)
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	(9.0)	(5.3)	(14.7)	(11.3)
Ending balance	324.6	399.6	324.6	399.6
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	0.0	0.0	0.0	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	$ 0.3	$ 0.1	$ 0.3	$ 0.1

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