Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2025

Fair Value Disclosures [Abstract]

Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis

The following tables present the financial instruments measured at fair value on a recurring basis at September 30, 2025 and December 31, 2024:

	September 30, 2025
Assets	Level 1		Level 2		Level 3		Total
Cash equivalents - money market funds	$	974.6	$	—	$	—	$	974.6
Fixed maturity securities
U.S. Treasuries	443.0		—		—		443.0
Agencies	—		6.8		—		6.8
Non-U.S. government	—		41.2		—		41.2
Corporate bonds	—		1,720.2		—		1,720.2
Residential mortgage-backed	—		250.6		—		250.6
Commercial mortgage-backed	—		0.6		—		0.6
Other asset-backed securities	—		248.1		—		248.1
Total fixed maturity securities	443.0		2,267.5		—		2,710.5
Short-term investments
Corporate bonds	—		0.2		—		0.2
U.S. Treasuries	187.7		—		—		187.7
Total short-term investments	187.7		0.2		—		187.9
Other assets
Investments pending settlement	4.6		—		—		4.6
Derivative assets	—		0.2		—		0.2
Total other assets	4.6		0.2		—		4.8
Total assets measured at fair value	$	1,609.9	$	2,267.9	$	—	$	3,877.8
Liabilities
Derivative liabilities	$	—	$	(0.1)	$	—	$	(0.1)
Investments pending settlement	(10.6)		—		—		(10.6)
Total other liabilities	(10.6)		(0.1)		—		(10.7)
Total liabilities measured at fair value	$	(10.6)	$	(0.1)	$	—	$	(10.7)

	December 31, 2024
Assets	Level 1		Level 2		Level 3		Total
Cash equivalents - money market funds	$	558.1	$	—	$	—	$	558.1
Fixed maturity securities
U.S. Treasuries	746.6		—		—		746.6
Agencies	—		11.5		—		11.5
Non-U.S. government	—		46.4		—		46.4
Corporate bonds	—		1,913.2		—		1,913.2
Residential mortgage-backed	—		279.1		—		279.1
Commercial mortgage-backed	—		0.4		—		0.4
Other asset-backed securities	—		414.4		—		414.4
Total fixed maturity securities	746.6		2,665.0		—		3,411.6
Short-term investments
Corporate bonds	—		1.1		—		1.1
U.S. Treasuries	220.7		—		—		220.7
Other asset-backed securities	—		0.3		—		0.3
Total short-term investments	220.7		1.4		—		222.1
Other assets
Investments pending settlement	0.5		—		—		0.5
Total other assets	0.5		—		—		0.5
Total assets measured at fair value	$	1,525.9	$	2,666.4	$	—	$	4,192.3
Liabilities
Other liabilities
Derivative liabilities	$	—	$	(0.5)	$	—	$	(0.5)
Investments pending settlement	(21.1)		—		—		(21.1)
Total other liabilities	(21.1)		(0.5)		—		(21.6)
Total liabilities measured at fair value	$	(21.1)	$	(0.5)	$	—	$	(21.6)

Schedule of Fair Value, by Balance Sheet Grouping

The following table presents financial instruments for which the carrying value differs from the estimated fair values at September 30, 2025 and December 31, 2024. The fair values of the below financial instruments are based on observable inputs and are considered Level 2 measurements.

	September 30, 2025				December 31, 2024
	Fair Value		Carrying Value		Fair Value		Carrying Value
7.750% Subordinated notes due 2055	$	427.3	$	393.4	$	—	$	—
4.875% Senior notes due 2030	330.6		326.2		319.7		325.6
6.625% Fixed Rate Reset Junior Subordinated notes due 2041	124.7		123.3		123.2		123.3
Preference securities	$	—	$	—	$	57.2	$	58.4