Credit Agreement and Term Loan Agreement - Additional Information (Detail) (USD $)	1 Months Ended			1 Months Ended						12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Feb. 28, 2013 Senior secured revolving credit agreement [Member]	Feb. 28, 2013 Senior secured revolving credit agreement [Member] Revolving Credit Facility [Member]	Feb. 28, 2013 Senior secured revolving credit agreement [Member] Minimum [Member]	Feb. 28, 2013 Senior secured revolving credit agreement [Member] Maximum [Member]	Feb. 28, 2013 Senior Secured Term Loan -Prudential Loan Agreement [Member]	Feb. 28, 2013 Senior Secured Revolving Credit Facility And Senior Secured Term Loan Facility [Member]	Dec. 31, 2012 Term loan [Member]	Feb. 28, 2013 LIBOR [Member] Senior secured revolving credit agreement [Member] Minimum [Member]	Feb. 28, 2013 LIBOR [Member] Senior secured revolving credit agreement [Member] Maximum [Member]
Line of Credit Facility [Line Items]
Senior secured revolving credit agreement		$ 175,000,000		$ 175,000,000
Amount of amended term loan agreement with TD Bank		25,000,000
Senior secured revolving credit agreement, expiration date										Mar. 31, 2013
Borrowings under credit agreement		150,290,000	147,700,000
Annual bearing interest rate for credit agreement		3.25%
Term loan borrowings		22,030,000	22,810,000
Interest rate of term loan		3.50%
Amortization of Loan Origination Costs	4,144,000
Senior secured revolving credit agreement, maturity date				Jul. 31, 2015				Feb. 28, 2021
Extension of credit agreement				1 year
Total bank syndicate commitment allocated to term loan				25,000,000
Total Bank Syndicate commitment allocated to a revolving facility				150,000,000
Revolving credit facility minimum					50,000,000
Revolving credit facility maximum					200,000,000
Credit agreement margin on borrowing base rate						1.50%	2.00%
Credit agreement borrowing Margin on LIBOR rate											2.50%	3.00%
Annual commitment fee on undrawn funds						0.30%	0.40%
Amount of rate increase in case of default				0.0200
Amount of rate increase in case of default								0.0200
Senior secured term loan, amount								100,000,000
Interest rate on prudential loan agreement								6.00%
Aggregate maturity of credit agreement and term loan, year 2015									71,900,000
Principal repayment of senior secured term loan in year eight									$ 100,000,000