Debt, Net - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Convertible Notes
Debt Instrument [Line Items]
Interest expense—stated coupon rate	$ 5,337	$ 3,642
Interest expense—amortization of debt discount	3,121	2,428
Total interest expense—Convertible Notes and Exchangeable Notes	8,458	6,070
Cabot senior secured notes | Senior Notes
Debt Instrument [Line Items]
Total interest expense—Convertible Notes and Exchangeable Notes	23,774	21,484
Interest expense | Cabot senior secured notes | Senior Notes
Debt Instrument [Line Items]
Interest expense—stated coupon rate	23,655	21,600
Interest expense—amortization of debt discount	$ 119	$ (116)