SUPPLEMENTAL QUARTERLY FINANCIAL DATA (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SUPPLEMENTAL QUARTERLY FINANCIAL DATA (Unaudited).
Operating revenue	$ 136,299	$ 131,325	$ 130,557	$ 107,755	$ 108,492	$ 103,955	$ 115,451	$ 116,220	$ 505,936	$ 444,118	$ 438,690
Total revenue	144,632	142,535	138,010	119,028	115,137	111,006	125,056	123,271	544,205	474,470	482,083
Gross profit	68,308	63,562	59,091	54,055	50,546	49,725	51,970	53,240	245,016	205,481
Net income (loss)	$ 8,750	$ (19,180)	$ (3,241)	$ 1,733	$ (630)	$ 5,010	$ (3,419)	$ (2,298)	$ (11,938)	$ (1,337)	$ 11,110
Basic (in dollars per share)	$ 0.24	$ (0.52)	$ (0.09)	$ 0.05	$ (0.02)	$ 0.14	$ (0.10)	$ (0.07)	$ (0.33)	$ (0.04)	$ 0.31
Diluted (in dollars per share)	$ 0.24	$ (0.52)	$ (0.09)	$ 0.05	$ (0.02)	$ 0.14	$ (0.10)	$ (0.07)	$ (0.33)	$ (0.04)	$ 0.30