Credit Agreement and Term Loan Agreement - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2013	Dec. 31, 2012
Line of Credit Facility [Line Items]
Senior secured revolving credit agreement		$ 175,000,000	$ 175,000,000
Amount of amended term loan agreement		100,000,000	25,000,000
Senior secured revolving credit agreement, expiration date		Aug. 31, 2015	Mar. 31, 2013
Amended term loan agreement, expiration date		Feb. 28, 2021	Mar. 31, 2013
Borrowings under credit agreement		71,900,000	150,290,000
Annual bearing interest rate for credit agreement			3.25%
Interest rate of term loan		6.00%	3.50%
Amortization of loan origination costs	4,144,000
Credit agreement initiation date		Feb. 25, 2013
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
Amount of rate increase in case of default		2.00%
Senior secured term loan, issuance date		Feb. 25, 2013
Amount of rate increase in case of default		2.00%
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
Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Option to increase credit facility		50,000,000
Maximum [Member]
Line of Credit Facility [Line Items]
Credit agreement margin on borrowing base rate		2.00%
Annual commitment fee on undrawn funds		0.40%
Maximum [Member] | Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Credit facility amount		200,000,000
Minimum [Member]
Line of Credit Facility [Line Items]
Credit agreement margin on borrowing base rate		1.50%
Annual commitment fee on undrawn funds		0.30%
Term loan [Member]
Line of Credit Facility [Line Items]
Borrowings outstanding			$ 22,030,000
LIBOR [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Credit agreement margin on borrowing base rate		3.00%
LIBOR [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Credit agreement margin on borrowing base rate		2.50%