QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data [Line Items]
Operating revenue	$ 176,878	$ 232,537	$ 179,031	$ 126,111	$ 167,417	$ 221,288	$ 174,938	$ 134,553	$ 714,557	$ 698,196	$ 676,113
Net operating income	20,356	51,567	27,013	476	26,536	47,329	26,839	9,836	99,412	110,540	107,083
Net income (loss)	$ 11,336	$ 42,424	$ 16,996	$ (7,640)	$ 15,368	$ 36,173	$ 14,805	$ (762)	$ 63,116	$ 65,584	$ 72,940
Diluted (loss) earnings per share (in dollars per share)	$ 0.24	$ 0.88	$ 0.35	$ (0.16)	$ 0.32	$ 0.75	$ 0.31	$ (0.02)	$ 1.31	$ 1.36	$ 1.52
Common stock market price range:
Dividends paid per common share (in dollars per share)	0.1975	0.1975	0.1975	0.1975	0.1875	0.1875	0.1875	0.1875	0.790	0.750	$ 0.720
Maximum
Common stock market price range:
Common stock market price (in dollars per share)	56.49	57.48	54.56	55.05	49.07	42.95	41.65	45.85	56.49	49.07
Minimum
Common stock market price range:
Common stock market price (in dollars per share)	$ 48.78	$ 49.52	$ 48.00	$ 44.60	$ 40.10	$ 38.85	$ 35.60	$ 35.25	$ 48.78	$ 40.10