Stock-based compensation and warrants - Summary of Restricted Stock Units(Detail) (Restricted Stock Units R S U [Member], USD $) In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Restricted Stock Units R S U [Member]
Shares
Unvested balance at beginning of period	179	179
Unvested balance at end of period	179	179
Weighted-average grant date fair value
Unvested balance at beginning of period	$ 30.47	$ 30.47
Unvested balance at end of period	$ 30.47	$ 30.47