Long-Term Debt and Short-Term Debt (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Summary of long-term debt and short-term debt
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.	$ 11,429
Total debt	63,658	69,516
Less current maturities of long-term debt	7,943	5,801
Long-term debt, excluding current maturities of long-term debt	55,715	63,715
October 26, 2017 [Member]
Summary of long-term debt and short-term debt
Borrowings under our $100,000 revolving credit facility bearing interest at a floating rate equal to LIBOR (0.25% at March 31,2013) plus an applicable margin of 1.50% at March 31, 2013, expiring October 26, 2017.	32,229	38,087
April 26, 2014 [Member]
Summary of long-term debt and short-term debt
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.	11,429	11,429
December 20, 2018 [Member]
Summary of long-term debt and short-term debt
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