Financial Instruments - Carrying Amounts and Estimated Fair Values of Notes (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
2023 notes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net Carrying Amount	$ 23,243	$ 22,482
Estimated Fair Value	38,980	59,894
2025 notes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net Carrying Amount	650,093	488,268
Estimated Fair Value	660,556	802,083
2027 notes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net Carrying Amount	562,674	0
Estimated Fair Value	$ 516,856	$ 0