Summary of Significant Accounting Policies - Fair Value Measurements (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security	Jan. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value transfers in or out of Level 1, Level 2 or Level 3	$ 0	$ 0
Securities in an unrealized gain or loss position | security	0	0
Discount on debt	$ (88,036,000)	$ (107,017,000)
Convertible Notes Payable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount on debt	$ (88,000,000.0)
Convertible Notes Payable | 1.75% convertible senior note due 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate (as a percent)	1.75%
Convertible notes ($450.0 face value)			$ 309,100,000
Reported Value Measurement | Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 532,800,000	487,400,000
Reported Value Measurement | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible notes ($450.0 face value)	356,300,000
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	532,800,000	487,400,000
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Convertible notes ($450.0 face value)	516,400,000
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 0	$ 0