Summary of Business and Significant Accounting Policies - Changes in Preferred Stock Warrant Obligation Measured and Recorded at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Conversion to common stock warrant	$ (745)
Significant unobservable inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balances at beginning of period	507	145
Change in unrealized loss included in earnings	238	124
Conversion to common stock warrant	(745)
Balances at end of period		$ 269