Debt - Interest Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Convertible Notes
Debt Instrument [Line Items]
Interest expense—stated coupon rate	$ 4,064	$ 3,297	$ 7,588	$ 6,608
Interest expense—amortization of debt discount (premium)	2,415	2,438	4,901	4,865
Total interest expense—convertible notes	6,479	5,735	12,489	11,473
Cabot Senior Secured Notes
Debt Instrument [Line Items]
Total interest expense—convertible notes	24,046	25,409	47,122	50,561
Interest expense | Cabot Senior Secured Notes
Debt Instrument [Line Items]
Interest expense—stated coupon rate	25,011	27,846	48,993	55,489
Interest expense—amortization of debt discount (premium)	116	257	226	384
Interest Income | Cabot Senior Secured Notes
Debt Instrument [Line Items]
Interest expense—amortization of debt discount (premium)	$ (1,081)	$ (2,694)	$ (2,097)	$ (5,312)