Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Current assets:
Trade accounts receivable, allowance for doubtful accounts	$ 2,699	$ 1,926
Property and equipment, accumulated depreciation	$ 120,231	$ 109,925
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)	31,097,862	30,766,517
Common stock, shares outstanding (in shares)	26,037,648	25,703,125
Treasury stock, shares of common stock (in shares)	5,060,214	5,063,392