Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses, Allowance for Credit Losses and Fair Value of Debt Securities Available-For-Sale (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 517.3	$ 557.3
Unrealized Gain	2.1	0.9
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	519.4	558.2
Short-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	283.6	270.5
Unrealized Gain	1.7	0.5
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	285.3	271.0
Short-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	128.9	142.3
Unrealized Gain	0.4	0.4
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	129.3	142.7
Short-term investments | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	104.8	144.5
Unrealized Gain	0.0	0.0
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	104.8	144.5
Long-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	180.3	299.3
Unrealized Gain	1.2	0.5
Unrealized Loss	(0.1)	(0.1)
Allowance for Credit Losses	0.0
Fair Value	181.4	299.7
Long-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	142.8	250.5
Unrealized Gain	1.2	0.5
Unrealized Loss	(0.1)	(0.1)
Allowance for Credit Losses	0.0
Fair Value	143.9	250.9
Long-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37.5	48.8
Unrealized Gain	0.0	0.0
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	$ 37.5	$ 48.8