Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

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

The following table presents the financial instruments measured at fair value on a recurring basis at December 31, 2023 and 2022:

	December 31, 2023
Assets	Level 1		Level 2		Level 3		Total
Cash equivalents - money market funds	$	557.9	$	—	$	—	$	557.9
Fixed maturity securities
U.S. Treasuries	686.7		—		—		686.7
Agencies	—		16.2		—		16.2
Non-U.S. government	—		76.0		—		76.0
Corporate bonds	—		1,787.3		—		1,787.3
Residential mortgage-backed	—		188.2		—		188.2
Commercial mortgage-backed	—		51.1		—		51.1
Other asset backed securities	—		439.4		—		439.4
Total fixed maturity securities	686.7		2,558.2		—		3,244.9
Short-term investments
U.S. Treasuries	49.0		—		—		49.0
Total short-term investments	49.0		—		—		49.0
Other investments*	46.9		—		—		46.9
Other assets
Investments pending settlement	2.2		—		—		2.2
Total other assets	2.2		—		—		2.2
Total assets measured at fair value	$	1,342.7	$	2,558.2	$	—	$	3,900.9
Liabilities
Other liabilities
Derivative liabilities	$	—	$	(1.1)	$	—	$	(1.1)
Total other liabilities	—		(1.1)		—		(1.1)
Total liabilities measured at fair value	$	—	$	(1.1)	$	—	$	(1.1)

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*excludes investments in the York Funds

	December 31, 2022
Assets	Level 1		Level 2		Level 3		Total
Cash equivalents - money market funds	$	685.2	$	—	$	—	$	685.2
Fixed maturity securities
U.S. Treasuries	615.8		—		—		615.8
Agencies	—		17.1		—		17.1
Non-U.S. government	—		110.9		—		110.9
Corporate bonds	—		1,020.3		—		1,020.3
Residential mortgage-backed	—		34.9		—		34.9
Commercial mortgage-backed	—		51.6		—		51.6
Other asset backed securities	—		200.3		—		200.3
Total fixed maturity securities	615.8		1,435.1		—		2,050.9
Short-term investments
Corporate bonds	—		5.4		—		5.4
Non-U.S. government	—		23.1		—		23.1
U.S. Treasuries	228.5		—		—		228.5
Total short-term investments	228.5		28.5		—		257.0
Other investments*	43.4		72.8		—		116.2
Other Assets
Derivative assets	—		6.3		—		6.3
Investment pending settlement	2.0		—		—		2.0
Total other assets	2.0		6.3		—		8.3
Total assets measured at fair value	$	1,574.9	$	1,542.7	$	—	$	3,117.6

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*excludes investments in the York Funds

Fair Value, by Balance Sheet Grouping

	December 31, 2023				December 31, 2022
	Fair Value		Carrying Value		Fair Value		Carrying Value
4.875% Senior notes due 2030	$	273.1	$	325.0	$	322.0	$	324.4
6.625% Fixed Rate Reset Junior Subordinated notes due 2041	121.6		123.2		115.5		123.1
Preference securities	$	55.6	$	58.4	$	54.4	$	58.4