Investments - Available-for-Sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,988.6	$ 3,771.7
Gross Unrealized Gains	1.4	7.4
Gross Unrealized Losses	(20.7)	(21.9)
Fair Value	2,969.3	3,757.2
Available-for-sale debt securities receivable		21.0
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,415.1	3,044.5
Gross Unrealized Gains	0.6	5.3
Gross Unrealized Losses	(17.1)	(17.2)
Fair Value	2,398.6	3,032.6
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	573.5	727.2
Gross Unrealized Gains	0.8	2.1
Gross Unrealized Losses	(3.6)	(4.7)
Fair Value	$ 570.7	$ 724.6