Debt - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Convertible notes
Debt Instrument [Line Items]
Interest expense—stated coupon rate	$ 4,117	$ 3,317	$ 11,705	$ 9,925
Interest expense—amortization of debt discount (premium)	2,473	2,501	7,374	7,366
Total interest expense—convertible notes	6,590	5,818	19,079	17,291
Cabot senior secured notes
Debt Instrument [Line Items]
Total interest expense—convertible notes	23,646	23,447	70,768	74,008
Interest expense | Cabot senior secured notes
Debt Instrument [Line Items]
Interest expense—stated coupon rate	24,285	25,870	73,278	81,359
Interest expense—amortization of debt discount (premium)	119	119	345	503
Interest Income | Cabot senior secured notes
Debt Instrument [Line Items]
Interest expense—amortization of debt discount (premium)	$ (758)	$ (2,542)	$ (2,855)	$ (7,854)