Employee benefit plans - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Employee Benefit Plans [Line Items]
Employee deferrals deemed vested percentage upon contribution			100.00%
Company's contributions to the plan	$ 200,000	$ 200,000	$ 600,000	$ 500,000
Company's matching contributions to the plan	$ 6,000	$ 6,000	$ 20,000	$ 6,000
Maximum [Member]
Employee Benefit Plans [Line Items]
Non-qualified deferred compensation plan allowable eligible participants deferral percentage of base salary			80.00%
Non-qualified deferred compensation plan allowable eligible participants deferral percentage of other compensation			100.00%