Long-Term Debt and Short-Term Debt - Summary of Long-Term Debt and Short-Term Debt (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Borrowings under our $40,000 aggregate principal amount of fixed rate notes bearing interest at a fixed rate of 4.89% maturing on April 26, 2014. Annual principal payments of $5,714 began on April 26, 2008 and extend through the date of maturity.	$ 5,714
Total debt	51,714	69,516
Less current maturities of long-term debt	5,714	5,801
Long-term debt, excluding current maturities of long-term debt	46,000	63,715
October 26, 2017 [Member]
Debt Instrument [Line Items]
Borrowings under our $100,000 revolving credit facility bearing interest at a floating rate equal to LIBOR (0.25% at June 30, 2013) plus an applicable margin of 1.50% at June 30, 2013, expiring October 26, 2017	26,000	38,087
April 26, 2014 [Member]
Debt Instrument [Line Items]
Borrowings under our $40,000 aggregate principal amount of fixed rate notes bearing interest at a fixed rate of 4.89% maturing on April 26, 2014. Annual principal payments of $5,714 began on April 26, 2008 and extend through the date of maturity.	5,714	11,429
December 20, 2018 [Member]
Debt Instrument [Line Items]
Borrowings under our $20,000 aggregate principal amount of fixed rate notes bearing interest at a fixed rate of 4.64% maturing on December 20, 2018. Annual principal payments of $4,000 will begin on December 22, 2014 and extend through the date of maturity	$ 20,000	$ 20,000