Fair Value Measurements - Reconciliation of Warrant Liability Measured at Fair Value on Recurring Basis (Detail) (Shares Underlying Warrants Outstanding [Member], USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Shares Underlying Warrants Outstanding [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at December 31, 2012	$ 374
Exercise of warrants	(38)
Change in fair value of warrant liability	317
Balance at June 30, 2013	$ 653