Borrowings - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Convertible Notes And Senior Notes
Debt Instrument [Line Items]
Interest expense—stated coupon rate	$ 5,799	$ 5,337
Interest expense—amortization of debt discount	3,044	3,121
Total interest expense	8,843	8,458
Term Loan Facility | Cabot senior secured notes
Debt Instrument [Line Items]
Total interest expense	19,502	19,596
Term Loan Facility | Cabot senior secured notes | Interest Expense
Debt Instrument [Line Items]
Interest expense—stated coupon rate	19,417	19,394
Interest expense—amortization of debt discount	$ 85	$ 202