Fair Value of Financial Instruments - Notes Payable (Details) - Convertible Senior Notes - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Aug. 31, 2019	Aug. 12, 2019	Jun. 30, 2015
2.25% Convertible Senior Notes due 2022
Fair value disclosures
Debt Instrument, Repurchased Face Amount			$ 215,000
Aggregate principal amount of notes issued	$ 120,699	$ 120,699			$ 335,700
2.25% Convertible Senior Notes due 2022 | Significant Other Observable Inputs (Level 2)
Fair value disclosures
Estimated fair value	126,400	141,300
0.75% Convertible Senior Notes due 2024
Fair value disclosures
Aggregate principal amount of notes issued	200,000	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	206,000	235,700
1.50% Convertible Senior Notes due 2026
Fair value disclosures
Aggregate principal amount of notes issued	200,000	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	$ 206,300	$ 240,100