Investments - Fair Value of Investments (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Fixed maturity securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 3,271.4	$ 2,160.8
Unrealized gains	22.4	0.0
Unrealized losses	(48.9)	(109.9)
Fair Value	3,244.9	2,050.9
Fixed maturity securities \| U.S. Treasuries
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	692.4	643.1
Unrealized gains	4.1	0.0
Unrealized losses	(9.8)	(27.3)
Fair Value	686.7	615.8
Fixed maturity securities \| Agencies
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	16.2	17.5
Unrealized gains	0.1	0.0
Unrealized losses	(0.1)	(0.4)
Fair Value	16.2	17.1
Fixed maturity securities \| Non-U.S. government
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	77.6	115.2
Unrealized gains	0.2	0.0
Unrealized losses	(1.8)	(4.3)
Fair Value	76.0	110.9
Fixed maturity securities \| Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1,798.4	1,078.9
Unrealized gains	14.7	0.0
Unrealized losses	(25.8)	(58.6)
Fair Value	1,787.3	1,020.3
Fixed maturity securities \| Residential mortgage-backed
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	192.0	41.4
Unrealized gains	2.0	0.0
Unrealized losses	(5.8)	(6.5)
Fair Value	188.2	34.9
Fixed maturity securities \| Commercial mortgage-backed
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	53.1	55.2
Unrealized gains	0.0	0.0
Unrealized losses	(2.0)	(3.6)
Fair Value	51.1	51.6
Fixed maturity securities \| Other asset backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	441.7	209.5
Unrealized gains	1.3	0.0
Unrealized losses	(3.6)	(9.2)
Fair Value	439.4	200.3
Short-Term Investments
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	49.0	257.0
Unrealized gains	0.0	0.1
Unrealized losses	0.0	(0.1)
Fair Value	49.0	257.0
Short-Term Investments \| U.S. Treasuries
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	49.0	228.4
Unrealized gains	0.0	0.1
Unrealized losses	0.0	0.0
Fair Value	$ 49.0	228.5
Short-Term Investments \| Non-U.S. government
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost		23.2
Unrealized gains		0.0
Unrealized losses		(0.1)
Fair Value		23.1
Short-Term Investments \| Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost		5.4
Unrealized gains		0.0
Unrealized losses		0.0
Fair Value		$ 5.4

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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