Revolving Credit Facility - Additional Information (Detail) (USD $)	2 Months Ended	10 Months Ended	12 Months Ended			1 Months Ended		12 Months Ended		0 Months Ended	12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended
	Dec. 31, 2012	Oct. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 03, 2011 Secured Revolving Credit Facility	Dec. 21, 2011 Revolving Credit Facility	Dec. 31, 2012 Revolving Credit Facility	Dec. 31, 2012 Subsequent Quarters	Mar. 03, 2011 Revolving Facility [Member]	Dec. 31, 2012 Revolving Facility [Member]	Dec. 31, 2011 Revolving Facility [Member]	Dec. 21, 2011 Revolving Facility [Member]	Dec. 31, 2012 Minimum Revolving Facility [Member]	Dec. 31, 2012 Minimum Revolving Facility [Member] Libor Plus Applicable Margin	Dec. 31, 2012 Minimum Revolving Facility [Member] Bank S Prime Rate Less Applicable Margin	Dec. 31, 2012 Maximum Revolving Facility [Member]	Dec. 31, 2012 Maximum Revolving Facility [Member] Libor Plus Applicable Margin	Dec. 31, 2012 Maximum Revolving Facility [Member] Bank S Prime Rate Less Applicable Margin	Dec. 31, 2012 December 31, 2012 amendment [Member]	Dec. 31, 2012 December 31, 2012 amendment [Member] Revolving Facility [Member]
Line of Credit Facility [Line Items]
Unsecured revolving credit facility, period										3 years
Maximum amount available under the Revolving Facility								$ 100,000,000		$ 100,000,000			$ 150,000,000
Maturity date						Mar. 03, 2014	Dec. 31, 2014
Current borrowing capacity								23,300,000
Borrowings under the Revolving Facility								65,000,000
Letter of credit outstanding								3,100,000
Variable interest rate														1.25%	1.25%	0.25%	1.75%	1.75%	1.00%
Weighted average interest rate												2.20%
Repayments of Long-term Debt			47,000,000	54,000,000	0			47,000,000
Proceeds from issuance of debt			27,000,000	139,000,000	0			27,000,000
Line of Credit Facility [Abstract]
2013	0		0
Long-term debt	65,000,000		65,000,000	85,000,000
Increase of interest rate on all obligations under the revolving facility in the event of default											2.00%
Fee on the maximum amount available to be drawn under each letter of credit														0.60%			1.00%
Percentage of fee to be paid on unused portion of the New Facility														0.15%			0.25%
Maximum funded debt to EBITDA											2.5										2.0
Asset coverage ratio											1.0
Minimum adjusted EBITDA allowed for the trailing twelve months	$ 40,000,000	$ 45,000,000							$ 45,000,000											$ 35,000,000