Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses, Allowance for Credit Losses and Fair Value of Debt Securities Available-For-Sale (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	$ 900.8	$ 781.7
Unrealized gain, current	0.1	0.3
Unrealized loss, current	(1.7)	(1.5)
Fair value, current	899.2	780.5
Amortized cost, noncurrent	639.8	684.6
Unrealized gain, noncurrent	0.4	3.4
Unrealized loss, noncurrent	(2.4)	(0.5)
Fair value, noncurrent	637.8	687.5
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	57.2	53.5
Unrealized gain, current	0.0	0.0
Unrealized loss, current	0.0	0.0
Fair value, current	57.2	53.5
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	530.9	382.1
Unrealized gain, current	0.1	0.1
Unrealized loss, current	(1.2)	(1.0)
Fair value, current	529.8	381.2
Amortized cost, noncurrent	476.4	483.5
Unrealized gain, noncurrent	0.4	2.9
Unrealized loss, noncurrent	(1.7)	(0.4)
Fair value, noncurrent	475.1	486.0
Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	312.7	346.1
Unrealized gain, current	0.0	0.2
Unrealized loss, current	(0.5)	(0.5)
Fair value, current	312.2	345.8
Amortized cost, noncurrent	163.4	201.1
Unrealized gain, noncurrent	0.0	0.5
Unrealized loss, noncurrent	(0.7)	(0.1)
Fair value, noncurrent	$ 162.7	$ 201.5