CONSOLIDATED SUPPLEMENTARY FINANCIAL DATA (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue:
Total revenue	$ 101,891	$ 99,134	$ 94,720	$ 97,351	$ 97,559	$ 89,855	$ 76,501	$ 72,682	$ 393,096	$ 336,597	$ 296,278
Operating costs and expenses:
Total operating costs and expenses	86,172	82,124	86,476	83,917	97,035	78,039	64,493	63,388	338,689	302,955	224,858
Operating income (loss)	15,719	17,010	8,244	13,434	524	11,816	12,008	9,294	54,407	33,642	71,420
Net Income (loss)	$ (36,168)	$ 7,695	$ 2,605	$ 3,006	$ (3,445)	$ 4,511	$ 3,876	$ 1,878	$ (22,862)	$ 6,820	$ 211,388
Net income (loss) per share:
Net income (loss) per share - basic (in dollars per share)	$ (0.33)	$ 0.07	$ 0.02	$ 0.03	$ (0.03)	$ 0.04	$ 0.04	$ 0.02	$ (0.21)	$ 0.06	$ 1.84
Net income (loss) per share - diluted (in dollars per share)	$ (0.33)	$ 0.07	$ 0.02	$ 0.03	$ (0.03)	$ 0.04	$ 0.03	$ 0.02	$ (0.21)	$ 0.06	$ 1.80
Shares used in per share calculations:
Basic (in shares)	109,737	109,555	110,060	111,464	110,788	110,214	109,904	109,733	110,198	110,162	114,814
Diluted (in shares)	109,737	113,119	112,565	115,325	110,788	113,723	112,061	112,252	110,198	113,140	117,484