Fair Value of Financial Instruments - Notes Payable (Details) - Convertible Senior Notes - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Aug. 31, 2019	Jun. 30, 2015
2.25% Convertible Senior Notes due 2022
Fair value disclosures
Aggregate principal amount of notes issued	$ 120,699	$ 120,699		$ 335,700
Debt redeemed/repurchased			$ 215,000
2.25% Convertible Senior Notes due 2022 | Significant Other Observable Inputs (Level 2)
Fair value disclosures
Estimated fair value	130,100	130,200
0.75% Convertible Senior Notes due 2024
Fair value disclosures
Aggregate principal amount of notes issued	200,000	200,000	200,000
0.75% Convertible Senior Notes due 2024 | Significant Other Observable Inputs (Level 2)
Fair value disclosures
Estimated fair value	220,900	222,300
1.50% Convertible Senior Notes due 2026
Fair value disclosures
Aggregate principal amount of notes issued	200,000	200,000	$ 200,000
1.50% Convertible Senior Notes due 2026 | Significant Other Observable Inputs (Level 2)
Fair value disclosures
Estimated fair value	$ 225,900	$ 224,100