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Reference 29: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
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-Section 50
-Paragraph 3
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Reference 30: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-Subparagraph (d)
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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-SubTopic 10
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Investments Available for Sale Securities (Details) - USD ($) $ in Millions	Jun. 30, 2026	Mar. 31, 2026	Dec. 31, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost	$ 2,054.5		$ 2,031.8
Debt securities, available-for-sale, gross unrealized gain	12.9		26.0
Debt securities, available-for-sale, gross unrealized loss	(28.4)		(16.7)
Fixed maturity securities, fair value	2,037.9		2,040.7
Total AFS, amortized cost	2,076.2		2,041.9
Total AFS, gross unrealized gain	12.9		26.0
Total AFS, gross unrealized loss	(28.4)		(16.7)
Short-term investments at fair value (amortized cost $21.7 at June 30, 2026 and $10.1 at December 31, 2025)	21.7		10.1
Total AFS investments	2,059.6		2,050.8
Debt Securities, Available-for-sale, Allowance for Credit Loss	(1.1)	$ (0.7)	(0.4)	$ (1.1)	$ (1.4)	$ (1.1)
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost	81.9		80.0
Debt securities, available-for-sale, gross unrealized gain	0.2		1.0
Debt securities, available-for-sale, gross unrealized loss	(1.2)		(0.9)
Fixed maturity securities, fair value	80.9		80.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0		0.0
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost	150.0		156.5
Debt securities, available-for-sale, gross unrealized gain	2.4		4.2
Debt securities, available-for-sale, gross unrealized loss	(0.7)		(0.8)
Fixed maturity securities, fair value	151.7		159.9
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0		0.0
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost	713.4		654.8
Debt securities, available-for-sale, gross unrealized gain	6.4		12.7
Debt securities, available-for-sale, gross unrealized loss	(17.0)		(12.2)
Fixed maturity securities, fair value	702.8		655.3
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0		0.0
Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost	753.0		800.5
Debt securities, available-for-sale, gross unrealized gain	2.5		4.8
Debt securities, available-for-sale, gross unrealized loss	(7.3)		(2.4)
Fixed maturity securities, fair value	748.2		802.9
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0		0.0
Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost	29.9		28.6
Debt securities, available-for-sale, gross unrealized gain	0.2		0.3
Debt securities, available-for-sale, gross unrealized loss	(0.1)		0.0
Fixed maturity securities, fair value	30.0		28.9
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0		0.0
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost	166.1		160.9
Debt securities, available-for-sale, gross unrealized gain	0.9		2.3
Debt securities, available-for-sale, gross unrealized loss	(0.7)		(0.1)
Fixed maturity securities, fair value	166.3		163.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0		0.0
Foreign Government Debt
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost	2.0		2.0
Debt securities, available-for-sale, gross unrealized gain	0.0		0.0
Debt securities, available-for-sale, gross unrealized loss	0.0		0.0
Fixed maturity securities, fair value	2.0		2.0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0		0.0
Collateralized Loan Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost	2.5		12.5
Debt securities, available-for-sale, gross unrealized gain	0.0		0.0
Debt securities, available-for-sale, gross unrealized loss	0.0		0.0
Fixed maturity securities, fair value	2.5		12.5
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0		0.0
Other Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost	155.7		136.0
Debt securities, available-for-sale, gross unrealized gain	0.3		0.7
Debt securities, available-for-sale, gross unrealized loss	(1.4)		(0.3)
Fixed maturity securities, fair value	153.5		136.0
Debt Securities, Available-for-sale, Allowance for Credit Loss	(1.1)		(0.4)
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Total AFS, amortized cost	21.7		10.1
Total AFS, gross unrealized gain	0.0		0.0
Total AFS, gross unrealized loss	0.0		0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0.0		$ 0.0