Financial Instruments - Carrying Amounts and Estimated Fair Values of Notes (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
1.75% Convertible Senior Notes due 2023
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net Carrying Amount	$ 23,428	$ 22,482
Estimated Fair Value	53,683	59,894
0% Convertible Senior Notes due 2025
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net Carrying Amount	648,713	488,268
Estimated Fair Value	772,651	802,083
0.5% Convertible Senior Notes Due 2027
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net Carrying Amount	488,261	0
Estimated Fair Value	$ 510,340	$ 0