Debt (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Summary of long-term debt and short-term debt
Borrowings under our $40,000 aggregate principal amount 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 long-term debt	69,516	78,132
Less current maturities of long-term debt	5,801	6,503
Long-term debt, excluding current maturities	63,715	71,629
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 December 31, 2012) plus an applicable margin of 1.50%, expiring October 26, 2017	38,087	40,989
April 26, 2014 [Member]
Summary of long-term debt and short-term debt
Borrowings under our $40,000 aggregate principal amount 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	17,143
December 20, 2018 [Member]
Summary of long-term debt and short-term debt
Borrowings under our $20,000 aggregate principal amount 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