Fair Value Disclosures [Abstract]

Fair Value, Investment Assets Measured on Recurring Basis

The following tables provide, for each hierarchy level, the Company’s investment assets that were measured at fair value on a recurring basis.


	DECEMBER 31, 2023
(in millions)	Total		Level 1		Level 2		Level 3
Fixed maturities:
U.S. Treasury and government agencies	$	462.6	$	324.3	$	138.3	$	—
Foreign government		2.2		—		2.2		—
Municipal		1,185.2		—		1,176.6		8.6
Corporate		3,849.7		—		3,849.6		0.1
Residential mortgage-backed, U.S. agency backed		1,168.7		—		1,168.7		—
Residential mortgage-backed, non-agency		142.2		—		142.2		—
Commercial mortgage-backed		818.2		—		811.2		7.0
Other asset-backed		356.5		—		356.5		—
Total fixed maturities		7,985.3		324.3		7,645.3		15.7
Equity securities		130.9		123.3		—		7.6
Other investments		3.8		—		—		3.8
Total investment assets at fair value	$	8,120.0	$	447.6	$	7,645.3	$	27.1


	DECEMBER 31, 2022
(in millions)	Total		Level 1		Level 2		Level 3
Fixed maturities:
U.S. Treasury and government agencies	$	420.2	$	282.7	$	137.5	$	—
Foreign government		2.2		—		2.2		—
Municipal		1,076.1		—		1,066.8		9.3
Corporate		3,729.9		—		3,729.8		0.1
Residential mortgage-backed, U.S. agency backed		974.0		—		974.0		—
Residential mortgage-backed, non-agency		99.8		—		99.8		—
Commercial mortgage-backed		836.4		—		828.9		7.5
Other asset-backed		343.2		—		343.2		—
Total fixed maturities		7,481.8		282.7		7,182.2		16.9
Equity securities		241.9		234.4		—		7.5
Other investments		3.6		—		—		3.6
Total investment assets at fair value	$	7,727.3	$	517.1	$	7,182.2	$	28.0

Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)

The following tables provide a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3).


YEAR ENDED DECEMBER 31, 2023	Fixed Maturities
(in millions)	Municipal			Corporate		Commercial mortgage- backed			Total			Equity and Other		Total Assets
Balance at beginning of year	$	9.3		$	0.1	$	7.5		$	16.9		$	11.1	$	28.0
Total gains (losses):
Included in net realized and unrealized investment gains (losses)		—			—		—			—			0.3		0.3
Included in other comprehensive income (loss) - changes in net unrealized gains on investment securities		0.2			—		0.1			0.3			—		0.3
Settlements		(0.9	)		—		(0.6	)		(1.5	)		—		(1.5	)
Balance at end of year	$	8.6		$	0.1	$	7.0		$	15.7		$	11.4	$	27.1
Changes in net unrealized gains for the period included in other comprehensive income (loss) for assets held at the end of the year	$	0.2		$	—	$	0.1		$	0.3		$	—	$	0.3


YEAR ENDED DECEMBER 31, 2022	Fixed Maturities
(in millions)	Municipal			Corporate		Commercial mortgage- backed			Total			Equity and Other			Total Assets
Balance at beginning of year	$	14.0		$	0.1	$	11.3		$	25.4		$	14.4		$	39.8
Total losses:
Included in net realized and unrealized investment gains (losses)		—			—		—			—			(3.3	)		(3.3	)
Included in other comprehensive income (loss) - changes in net unrealized losses on investment securities		(1.1	)		—		(1.1	)		(2.2	)		—			(2.2	)
Settlements		(3.6	)		—		(2.7	)		(6.3	)		—			(6.3	)
Balance at end of year	$	9.3		$	0.1	$	7.5		$	16.9		$	11.1		$	28.0
Changes in net unrealized losses for the period included in other comprehensive income (loss) for assets held at the end of the year	$	(1.1	)	$	—	$	(1.1	)	$	(2.2	)	$	—		$	(2.2	)