Financial Instruments - Carrying Amounts and Estimated Fair Values of Notes (Details) - USD ($) $ in Thousands	Jun. 30, 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	50,106	59,894
0% Convertible Senior Notes due 2025
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net Carrying Amount	649,403	488,268
Estimated Fair Value	747,808	802,083
0.5% Convertible Senior Notes Due 2027
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net Carrying Amount	562,114	0
Estimated Fair Value	$ 571,107	$ 0