Short-term and long-term debt - Additional Information (Detail) (USD $)	12 Months Ended											12 Months Ended
	Dec. 31, 2012	Jun. 22, 2012	Dec. 31, 2011	Dec. 31, 2012 Domestic Rate Loans	Dec. 31, 2012 Domestic Rate Loans Federal Funds Base Rate	Dec. 31, 2012 Domestic Rate Loans Libor Rate	Dec. 31, 2012 Eurodollar Rates	Dec. 31, 2012 Term Loan	Jun. 22, 2012 Term Loan	Dec. 31, 2012 Revolving Credit Facility	Jun. 22, 2012 Revolving Credit Facility	Dec. 31, 2012 Mortgage Note	Jun. 01, 2012 Mortgage Note	Dec. 31, 2012 Vehicle And Equipment Loans Minimum	Dec. 31, 2012 Vehicle And Equipment Loans Maximum
Debt Disclosure [Line Items]
Maximum borrowing under PNC Facility		$ 27,000,000						$ 40,000,000	$ 4,000,000	$ 36,000,000	$ 23,000,000		$ 855,000
Proceeds from borrowings under PNC facility	9,548,000
Fees and expenses relating to PNC facility	245,000
Borrowings outstanding under PNC facility	16,451,000							4,000,000
Available borrowing under PNC Facility	10,357,000
PNC Facility effective Rate of interest	4.00%												4.00%	2.90%	8.70%
Interest rate Description under PNC Facility	Interest charges with respect to loans are computed on the actual principal amount of loans outstanding during the month at a rate per annum equal to (A) with respect to domestic rate loans, the sum of (i) a rate per annum equal to the higher of (1) the base commercial lending rate of PNC, (2) the federal funds open rate plus .5% and (3) the daily LIBOR plus 1%, plus (ii) .5% and (B) with respect to Eurodollar rate loans, the sum of the Eurodollar rate plus 2.25%.
PNC facility Spread Rate for calculation of Interest				0.50%	0.50%	1.00%	2.25%
PNC credit Facility expiration date												Jun. 01, 2017
Capital leased assets gross	128,000
Principal balance of this mortgage note	$ 764,000		$ 903,000