QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Data [Line Items]
Operating revenue	$ 167,417	$ 221,288	$ 174,938	$ 134,553	$ 164,296	$ 214,037	$ 173,438	$ 124,342	$ 698,196	$ 676,113	$ 609,370
Net operating income	26,536	47,329	26,839	9,836	23,077	43,902	28,111	11,993	$ 110,540	$ 107,083	$ 82,636
Net income (loss)	$ 15,368	$ 36,173	$ 14,805	$ (762)	$ 15,108	$ 35,289	$ 19,971	$ 2,572
Earnings per share—diluted (in dollars per share)	$ 0.32	$ 0.75	$ 0.31	$ (0.02)	$ 0.31	$ 0.74	$ 0.42	$ 0.05	$ 1.36	$ 1.52	$ 1.01
Dividends paid per common share (in dollars per share)	0.1875	0.1875	0.1875	0.1875	0.18	0.18	0.18	0.18	0.750	0.720	$ 0.690
Maximum
Common stock market price range:
Common stock market price (in dollars per share)	49.07	42.95	41.65	45.85	46.15	39.65	39.40	37.6	49.07	46.15
Minimum
Common stock market price range:
Common stock market price (in dollars per share)	$ 40.1	$ 38.85	$ 35.6	$ 35.25	$ 38.15	$ 36.30	$ 32.75	$ 32.45	$ 40.1	$ 38.15
As Previously Reported
Quarterly Financial Data [Line Items]
Operating revenue		$ 218,983	$ 172,632	$ 132,247	$ 161,991	$ 211,731	$ 171,132	$ 122,036		$ 666,890
Net operating income		45,548	25,056	8,053	21,638	42,462	26,671	10,553		$ 94,623	$ 76,194
Net income (loss)		$ 34,392	$ 13,022	$ (2,545)	$ 13,669	$ 33,849	$ 18,531	$ 1,132
Earnings per share—diluted (in dollars per share)		$ 0.72	$ 0.27	$ (0.05)	$ 0.29	$ 0.70	$ 0.39	$ 0.02		$ 1.40
Dividends paid per common share (in dollars per share)		0.1875	0.1875	0.1875	0.18	0.18	0.18	0.18
As Previously Reported | Maximum
Common stock market price range:
Common stock market price (in dollars per share)		42.95	41.65	45.85	46.15	39.65	39.40	37.6		46.15
As Previously Reported | Minimum
Common stock market price range:
Common stock market price (in dollars per share)		$ 38.85	$ 35.6	$ 35.25	$ 38.15	$ 36.30	$ 32.75	$ 32.45		$ 38.15