Investments - Summary of Amortized Cost, Gross Unrealized Gains and Losses, Fair Value and Level in Fair Value Hierarchy for Investments (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 58.2	$ 162.0
Gross Unrealized Gains	0.4	0.4
Gross Unrealized Losses	0.0	0.0
Fair Value	58.6	162.4
Short-term investments | Level 1 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	53.1	85.0
Short-term investments | Level 2 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5.5	77.4
Short-term investments | U.S. government and agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	52.7	94.7
Gross Unrealized Gains	0.4	0.3
Gross Unrealized Losses	0.0	0.0
Fair Value	53.1	95.0
Short-term investments | U.S. government and agency bonds | Level 1 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	53.1	85.0
Short-term investments | U.S. government and agency bonds | Level 2 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0.0	10.0
Short-term investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	2.8	51.0
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	0.0	0.0
Fair Value	2.8	51.1
Short-term investments | Corporate bonds | Level 1 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0.0	0.0
Short-term investments | Corporate bonds | Level 2 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2.8	51.1
Short-term investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	2.7	6.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	2.7	6.3
Short-term investments | Certificates of deposit | Level 1 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0.0	0.0
Short-term investments | Certificates of deposit | Level 2 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2.7	6.3
Short-term investments | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		10.0
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		10.0
Short-term investments | Commercial paper | Level 1 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		0.0
Short-term investments | Commercial paper | Level 2 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		10.0
Long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	0.5	58.4
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	0.0	(0.1)
Fair Value	0.5	58.4
Long-term investments | Level 1 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0.0	42.9
Long-term investments | Level 2 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0.5	15.5
Long-term investments | U.S. government and agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		52.9
Gross Unrealized Gains		0.1
Gross Unrealized Losses		(0.1)
Fair Value		52.9
Long-term investments | U.S. government and agency bonds | Level 1 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		42.9
Long-term investments | U.S. government and agency bonds | Level 2 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		10.0
Long-term investments | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		2.8
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		2.8
Long-term investments | Corporate bonds | Level 1 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		0.0
Long-term investments | Corporate bonds | Level 2 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		2.8
Long-term investments | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	0.5	2.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	0.5	2.7
Long-term investments | Certificates of deposit | Level 1 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0.0	0.0
Long-term investments | Certificates of deposit | Level 2 | Recurring fair value measurements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 0.5	$ 2.7