Note 18 - Quarterly Financial Data (Unaudited) - Summary of Unaudited Quarterly Operating Results (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	[1]	Mar. 31, 2018	Dec. 31, 2017	[2]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues	$ 155,828	$ 154,246	$ 146,791		$ 141,425	$ 148,493		$ 125,673	$ 133,313	$ 113,733	$ 598,290	$ 521,212	$ 489,720
Operating income	27,848	24,231	(763)		11,922	19,266		12,861	14,361	3,943	63,238	50,431	57,669
Net income	$ 33,391	$ 18,574	$ (3,367)		$ 6,391	$ 25,723		$ 6,372	$ 6,517	$ 1,442	$ 54,989	$ 40,054	$ 37,636
Net income (loss) per common share - basic (in dollars per share)	$ 0.40	$ 0.22	$ (0.04)		$ 0.08	$ 0.30		$ 0.08	$ 0.08	$ 0.02	$ 0.66	$ 0.48	$ 0.45
Net income (loss) per common share - diluted (in dollars per share)	$ 0.38	$ 0.22	$ (0.04)		$ 0.07	$ 0.30		$ 0.07	$ 0.08	$ 0.02	$ 0.64	$ 0.47	$ 0.43

[1] Second quarter 2018 results include the Company's entry into the Settlement Agreement for the payment of $20 million, as described in Note 14.

[2] Fourth quarter 2017 results include a non-cash tax benefit of $15.1 million due to the revaluation of the Company’s deferred tax balances pursuant to the tax rate reduction included in the 2017 Tax Act.