Interest Expense - Summary of Interest Expense (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended				12 Months Ended
	Mar. 31, 2013	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Interest Income (Expense) [Line Items]
Issuance of Series C preferred stock warrants in connection with senior convertible notes								$ 10,677
Debt closing costs	$ 900						$ 47	585
Imputed interest on Cabot obligation								391
Other interest						$ 182	223	250
Total interest expense		$ 100	$ 100	$ 34,000	$ 16,200	$ 182	50,281	30,599
Subordinated Notes [Member]
Interest Income (Expense) [Line Items]
Changes in fair value							1,543	3,771
Issuance of Series C preferred stock warrants in connection with senior convertible notes								10,677
Senior Convertible Notes [Member]
Interest Income (Expense) [Line Items]
Changes in fair value							11,373	5,588
Convertible Notes, Net of Capitalization [Member]
Interest Income (Expense) [Line Items]
Changes in fair value							$ 37,095	$ 9,337