Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current assets:
Trade accounts receivable, allowance for doubtful accounts	$ 2,078	$ 2,052
Property and equipment, accumulated depreciation	$ 114,654	$ 110,772
Stockholders' equity:
Common stock, par value (in dollars per share)	$ 0.008	$ 0.008
Common stock, shares authorized (in shares)	40,000,000	40,000,000
Common stock, shares issued (in shares)	31,278,911	31,172,666
Common stock, shares outstanding (in shares)	26,188,831	26,113,544
Treasury stock, shares of common stock (in shares)	5,090,080	5,059,122