Investments - Available-for-Sale Debt Securities in Unrealized Loss Position (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Fair Value
Less than 12 months	$ 1,996.9	$ 1,311.2
12 months or longer	614.3	1,122.2
Total	2,611.2	2,433.4
Gross Unrealized Losses
Less than 12 months	(10.9)	(4.9)
12 months or longer	(9.8)	(17.0)
Total	(20.7)	(21.9)
U.S. government and agency securities
Fair Value
Less than 12 months	1,704.5	1,101.8
12 months or longer	492.9	838.1
Total	2,197.4	1,939.9
Gross Unrealized Losses
Less than 12 months	(9.7)	(4.4)
12 months or longer	(7.4)	(12.8)
Total	(17.1)	(17.2)
Corporate debt securities
Fair Value
Less than 12 months	292.4	209.4
12 months or longer	121.4	284.1
Total	413.8	493.5
Gross Unrealized Losses
Less than 12 months	(1.2)	(0.5)
12 months or longer	(2.4)	(4.2)
Total	$ (3.6)	$ (4.7)

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