Investments - Amortized Cost and Fair Values of Investment in Fixed Maturities (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 622.6	$ 607.1
Gross Unrealized Gains	2.2	1.6
Gross Unrealized Losses	(0.3)	(1.3)
Fair Value	624.5	607.4
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	486.7	470.6
Gross Unrealized Gains	1.7	1.3
Gross Unrealized Losses	(0.3)	(1.1)
Fair Value	488.1	470.8
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	87.6	107.6
Gross Unrealized Gains	0.4	0.3
Gross Unrealized Losses	0.0	(0.2)
Fair Value	88.0	107.7
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	33.9	22.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	33.9	22.9
Non-U.S. government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	14.4	6.0
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 14.5	$ 6.0