Investments - Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 623.5	$ 632.0
Gross Unrealized Gains	0.7	1.7
Gross Unrealized Losses	(4.4)	(6.3)
Fair Value	619.8	627.4
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	454.6	453.6
Gross Unrealized Gains	0.6	1.3
Gross Unrealized Losses	(3.5)	(5.0)
Fair Value	451.7	449.9
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	164.4	176.8
Gross Unrealized Gains	0.1	0.4
Gross Unrealized Losses	(0.9)	(1.3)
Fair Value	163.6	175.9
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	4.5	1.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 4.5	$ 1.6