QUARTERLY FINANCIAL DATA - (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
QUARTERLY FINANCIAL DATA - (UNAUDITED)
Net service revenues	$ 86,554	$ 85,128	$ 86,892	$ 89,950	$ 89,331	$ 86,207	$ 81,718	$ 82,596	$ 348,524	$ 339,853	$ 335,295
Gross margin	41,303	40,610	41,596	44,191	44,205	42,864	42,106	43,612	167,700	172,787	182,750
Net income	$ 3,704	$ 4,099	$ 4,549	$ 4,932	$ 5,305	$ 4,842	$ 4,950	$ 5,705	$ 17,284	$ 20,802	$ 30,713
Average shares outstanding
Basic (in shares)	9,280	9,256	9,255	9,268	9,296	9,296	9,284	9,205	9,285	9,278	9,123
Diluted (in shares)	9,313	9,315	9,315	9,334	9,328	9,346	9,376	9,329	9,324	9,360	9,352
Net income per share
Basic (in dollars per share)	$ 0.40	$ 0.44	$ 0.49	$ 0.53	$ 0.57	$ 0.52	$ 0.53	$ 0.62	$ 1.86	$ 2.24	$ 3.37
Diluted (in dollars per share)	$ 0.40	$ 0.44	$ 0.49	$ 0.53	$ 0.57	$ 0.52	$ 0.53	$ 0.61	$ 1.85	$ 2.22	$ 3.28