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Income taxes - Additional Information (Detail) (USD $)	12 Months Ended
Income taxes - Additional Information (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Income Tax [Line Items]
Income tax expense	$ 1,395,000	$ 634,000
Operating loss carryforwards	11,000,000
Operating loss carryforwards, expiration dates	expiring through 2029
Operating loss carryforwards, limitations on use	Company's NOLs are subject to annual limitations of $1,300,000
State and local income tax expense (benefit), continuing operations	5,395,000
Deferred tax assets	$ 4,000,000

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible state and local operating loss carryforwards.

The sum of the current income tax expense or benefit and the deferred income tax expense or benefit pertaining to continuing operations.