SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Effect of Topic 606 Adoption on Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue	$ 30,330	$ 29,586	$ 29,577	$ 29,733	$ 26,863	$ 27,004	$ 27,083	$ 27,855	$ 119,226	$ 108,805	$ 113,167
Operating costs	23,964	21,764	21,330	22,719	22,753	19,788	17,967	18,819	89,777	79,327	78,705
Operating income	6,366	7,822	8,247	7,014	4,110	7,216	9,116	9,036	29,449	29,478	34,462
Other income, net									16	(330)	(85)
Income before income taxes									18,463	19,402	25,310
Benefit from income taxes									(2,669)	(1,929)	(247)
Net income	$ 1,892	$ 5,928	$ 6,712	$ 6,600	$ 2,352	$ 5,275	$ 8,641	$ 5,063	$ 21,132	21,331	25,557
As reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue										111,336	111,125
Operating costs										80,223	78,485
Operating income										31,113	32,640
Other income, net										(308)	(10)
Income before income taxes										21,015	23,413
Benefit from income taxes										(1,192)	(759)
Net income										22,207	24,172
Adjustments | Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue										(2,531)	2,042
Operating costs										(896)	220
Operating income										(1,635)	1,822
Other income, net										(22)	(75)
Income before income taxes										(1,613)	1,897
Benefit from income taxes										(737)	512
Net income										$ (876)	$ 1,385