Fair Value of Financial Instruments - Transfers Between Different Fair Value Levels (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Change in net unrealized investment gains (losses) on fixed maturity securities	Change in net unrealized investment gains (losses) on fixed maturity securities
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net investment losses	Net investment losses
Financial Assets
Beginning balance	$ 371.4	$ 355.1
Transfers into Level 3	193.8	172.6
Transfers out of Level 3	(42.8)	(83.6)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(3.4)	(8.1)
Net unrealized investment gains (losses) included in OCI	(38.2)	6.3
Purchases	33.2	0.0
Issuances	0.0	0.0
Sales	(4.8)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(83.9)	(70.9)
Ending balance	425.3	371.4
Financial Liabilities
Beginning balance	106.6	104.5
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	(12.9)	10.0
Net unrealized investment gains (losses) included in OCI	0.0	0.0
Purchases	0.0	0.0
Issuances	7.4	4.9
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(10.1)	(12.8)
Ending balance	91.0	106.6
Municipal bonds
Financial Assets
Beginning balance	60.8	59.6
Transfers into Level 3	0.6	18.6
Transfers out of Level 3	(3.2)	0.0
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	(10.5)	(2.5)
Purchases	0.2	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	6.5	(14.9)
Ending balance	54.4	60.8
Corporate bonds
Financial Assets
Beginning balance	210.3	155.8
Transfers into Level 3	157.9	131.7
Transfers out of Level 3	(34.8)	(64.4)
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	(16.1)	0.0
Purchases	20.2	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(76.2)	(12.8)
Ending balance	261.3	210.3
Other asset-backed securities
Financial Assets
Beginning balance	98.9	139.4
Transfers into Level 3	34.5	21.3
Transfers out of Level 3	(4.8)	(19.2)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(3.3)	(8.2)
Net unrealized investment gains (losses) included in OCI	(11.6)	8.8
Purchases	12.8	0.0
Issuances	0.0	0.0
Sales	(4.8)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(14.1)	(43.2)
Ending balance	107.6	98.9
Total
Financial Assets
Beginning balance	370.0	354.8
Transfers into Level 3	193.0	171.6
Transfers out of Level 3	(42.8)	(83.6)
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(3.3)	(8.2)
Net unrealized investment gains (losses) included in OCI	(38.2)	6.3
Purchases	33.2	0.0
Issuances	0.0	0.0
Sales	(4.8)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(83.8)	(70.9)
Ending balance	423.3	370.0
Equity securities
Financial Assets
Beginning balance	1.4	0.3
Transfers into Level 3	0.8	1.0
Transfers out of Level 3	0.0	0.0
Total gains or losses
Net investment gains (losses) included in net income related to financial assets	(0.1)	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	(0.1)	0.0
Ending balance	$ 2.0	$ 1.4

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Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy.

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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

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Amount of issuances of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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