Commitments and contingencies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Commitments and Contingencies [Line Items]
Rent expense	$ 650,000	$ 601,000
Remediation costs	102,000	86,000
Officers' compensation	288,500
Insurance policy, amount	$ 1,000,000