QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Data [Line Items]
Operating revenue	$ 161,991	$ 211,731	$ 171,132	$ 122,036	$ 150,930	$ 184,268	$ 152,445	$ 121,727	$ 666,890	$ 609,370	$ 588,368
Net operating income	20,118	41,196	25,259	8,050	21,335	30,055	18,534	6,270	$ 94,623	$ 76,194	$ 71,153
Net income (loss)	$ 13,669	$ 33,849	$ 18,531	$ 1,132	$ 15,090	$ 22,875	$ 11,508	$ (798)
Earnings per share—diluted (in dollars per share)	$ 0.29	$ 0.70	$ 0.39	$ 0.02	$ 0.31	$ 0.48	$ 0.24	$ (0.02)	$ 1.40	$ 1.01	$ 0.94
Dividends paid per common share (in dollars per share)	0.18	0.18	0.18	0.18	0.1725	0.1725	0.1725	0.1725	0.720	0.690	$ 0.670
Maximum
Common stock market price range:
Common stock market price (in dollars per share)	46.15	39.65	39.40	37.6	36.85	35.62	34.95	27.33	46.15	36.85
Minimum
Common stock market price range:
Common stock market price (in dollars per share)	$ 38.15	$ 36.30	$ 32.75	$ 32.45	$ 29.25	$ 29.93	$ 26.22	$ 22.48	$ 38.15	$ 29.25