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Summary of Significant Accounting Policies - Fair Value Measurements (Details)	6 Months Ended
	Jun. 30, 2021 USD ($) security	Jun. 30, 2020 USD ($)	May 31, 2021 USD ($)	Dec. 31, 2020 USD ($) security	Jan. 31, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [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
Unamortized portion of debt discount	$ 238,634,000			$ 88,036,000
Unamortized debt issuance expense	$ 12,621,000			5,646,000
0.75 convertible senior notes due 2028 | Convertible Notes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Interest rate (as a percent)	0.75%
Convertible senior notes			$ 371,600,000
Unamortized portion of debt discount	$ 199,500,000
Unamortized debt issuance expense	$ 10,100,000		$ 16,000,000.0
1.75 convertible senior notes due 2025 | Convertible Notes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Interest rate (as a percent)	1.75%
Convertible senior notes					$ 309,100,000
Unamortized portion of debt discount	$ 39,100,000
Unamortized debt issuance expense	2,500,000				$ 14,200,000
Recurring basis | Carrying Value
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash and cash equivalents	928,300,000			532,800,000
Recurring basis | Carrying Value | Level 2 | 0.75 convertible senior notes due 2028
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Convertible senior notes	365,400,000
Recurring basis | Carrying Value | Level 2 | 1.75 convertible senior notes due 2025
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Convertible senior notes	183,400,000
Recurring basis | Fair Value | Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash and cash equivalents	928,300,000			532,800,000
Recurring basis | Fair Value | Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash and cash equivalents	0			0
Recurring basis | Fair Value | Level 2 | 0.75 convertible senior notes due 2028
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Convertible senior notes	641,100,000
Recurring basis | Fair Value | Level 2 | 1.75 convertible senior notes due 2025
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Convertible senior notes	236,300,000
Recurring basis | Fair Value | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash and cash equivalents	$ 0			$ 0

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