Investments - Amortized Cost and Fair Values of Investment in Fixed Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 679.3	$ 607.1
Gross Unrealized Gains	2.6	1.6
Gross Unrealized Losses	(0.1)	(1.3)
Fair Value	681.8	607.4
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	488.7	470.6
Gross Unrealized Gains	2.1	1.3
Gross Unrealized Losses	(0.1)	(1.1)
Fair Value	490.7	470.8
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	132.1	107.6
Gross Unrealized Gains	0.3	0.3
Gross Unrealized Losses	0.0	(0.2)
Fair Value	132.4	107.7
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	43.4	22.9
Gross Unrealized Gains	0.2	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	43.6	22.9
Non-U.S. government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	15.1	6.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 15.1	$ 6.0