Fair Value Measurements - Summary of Warrant Liabilities Measured at Fair Value on a Recurring Basis Using Unobservable Inputs (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value of preferred stock warrants exercised			$ 1,100
Change in fair value of preferred stock warrants	$ 8,630	$ 347	9,956	$ 496
Level 3 [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance as of December 31, 2014			1,810
Fair value of preferred stock warrants exercised			(1,125)
Change in fair value of preferred stock warrants			9,956
Balance as of June 30, 2015	$ 10,641		$ 10,641