Debt - Interest Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Convertible Notes [Member]
Debt Instrument [Line Items]
Interest expense—stated coupon rate	$ 3,297	$ 3,308	$ 6,608	$ 6,600
Interest expense—amortization of debt discount (premium)	2,438	2,310	4,865	4,588
Total interest expense—convertible notes	5,735	5,618	11,473	11,188
Cabot Senior Secured Notes [Member]
Debt Instrument [Line Items]
Total interest expense—convertible notes	25,409	21,491	50,561	42,794
Interest Expense [Member] | Cabot Senior Secured Notes [Member]
Debt Instrument [Line Items]
Interest expense—stated coupon rate	27,846	24,151	55,489	48,001
Interest expense—amortization of debt discount (premium)	257	0	384	0
Interest Income [Member] | Cabot Senior Secured Notes [Member]
Debt Instrument [Line Items]
Interest expense—amortization of debt discount (premium)	$ (2,694)	$ (2,660)	$ (5,312)	$ (5,207)