Investments - Amortized Cost and Fair Values of Investment in Fixed Maturities (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 772.4	$ 706.0
Gross Unrealized Gains	0.1	2.8
Gross Unrealized Losses	(4.0)	0.0
Fair Value	768.5	708.8
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	503.4	499.2
Gross Unrealized Gains	0.1	2.0
Gross Unrealized Losses	(2.1)	0.0
Fair Value	501.4	501.2
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	157.5	139.5
Gross Unrealized Gains	0.0	0.6
Gross Unrealized Losses	(1.5)	0.0
Fair Value	156.0	140.1
Asset-backed securities and collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	82.5	45.3
Gross Unrealized Gains	0.0	0.2
Gross Unrealized Losses	(0.3)	0.0
Fair Value	82.2	45.5
Non-U.S. government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	29.0	22.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	0.0
Fair Value	$ 28.9	$ 22.0