Debt - Interest Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Convertible Notes [Member]
Debt Instrument [Line Items]
Interest expense—stated coupon rate	$ 3,311	$ 3,292
Interest expense—amortization of debt discount (premium)	2,427	2,278
Total interest expense—convertible notes	5,738	5,570
Cabot Senior Secured Notes [Member]
Debt Instrument [Line Items]
Total interest expense—convertible notes	25,152	21,303
Interest Expense [Member] | Cabot Senior Secured Notes [Member]
Debt Instrument [Line Items]
Interest expense—stated coupon rate	27,643	23,850
Interest expense—amortization of debt discount (premium)	127	0
Interest Income [Member] | Cabot Senior Secured Notes [Member]
Debt Instrument [Line Items]
Interest expense—amortization of debt discount (premium)	$ (2,618)	$ (2,547)