Investments - Continuous Loss Position (Details) $ in Millions	Sep. 30, 2025 USD ($) security	Dec. 31, 2024 USD ($) security
Debt Securities, Available-for-Sale [Line Items]
Fair value	$ 127.6	$ 1,035.5
Gross unrealized losses	(0.3)	(7.6)
Gross unrealized losses	$ (0.2)	$ (1.2)
Number of securities \| security	109	431
U.S. Treasuries
Debt Securities, Available-for-Sale [Line Items]
Fair value	$ 64.2	$ 307.6
Gross unrealized losses	(0.1)	(2.8)
Gross unrealized losses	$ (0.1)	$ (0.3)
Number of securities \| security	28	48
Agencies
Debt Securities, Available-for-Sale [Line Items]
Fair value		$ 4.0
Gross unrealized losses		0.0
Gross unrealized losses		$ 0.0
Number of securities \| security		2
Non-U.S. government
Debt Securities, Available-for-Sale [Line Items]
Fair value	$ 1.2	$ 23.7
Gross unrealized losses	0.0	(0.1)
Gross unrealized losses	$ 0.0	$ (0.2)
Number of securities \| security	1	11
Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Fair value	$ 25.1	$ 543.6
Gross unrealized losses	(0.1)	(3.4)
Gross unrealized losses	$ (0.1)	$ (0.6)
Number of securities \| security	50	310
Residential mortgage-backed
Debt Securities, Available-for-Sale [Line Items]
Fair value	$ 10.0	$ 91.9
Gross unrealized losses	0.0	(1.1)
Gross unrealized losses	$ 0.0	$ (0.1)
Number of securities \| security	3	35
Commercial mortgage-backed
Debt Securities, Available-for-Sale [Line Items]
Fair value	$ 0.4
Gross unrealized losses	0.0
Gross unrealized losses	$ 0.0
Number of securities \| security	1
Other asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Fair value	$ 26.7	$ 64.7
Gross unrealized losses	(0.1)	(0.2)
Gross unrealized losses	$ 0.0	$ 0.0
Number of securities \| security	26	25

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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