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	Jun. 30, 2020	Dec. 31, 2019
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 530.3	$ 557.3
Unrealized Gain	2.9	0.9
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	533.2	558.2
Short-term investments | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	140.0	144.5
Unrealized Gain	0.1	0.0
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	140.1	144.5
Short-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	287.8	270.5
Unrealized Gain	2.0	0.5
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	289.8	271.0
Short-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	102.5	142.3
Unrealized Gain	0.8	0.4
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	103.3	142.7
Long-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	193.5	299.3
Unrealized Gain	1.7	0.5
Unrealized Loss	0.0	(0.1)
Allowance for Credit Losses	0.0
Fair Value	195.2	299.7
Long-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	173.5	250.5
Unrealized Gain	1.7	0.5
Unrealized Loss	0.0	(0.1)
Allowance for Credit Losses	0.0
Fair Value	175.2	250.9
Long-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20.0	48.8
Unrealized Gain	0.0	0.0
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	$ 20.0	$ 48.8