QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating revenue	$ 137,384	$ 191,184	$ 158,416	$ 110,515	$ 133,700	$ 184,404	$ 154,555	$ 111,444	$ 597,499	$ 584,103	$ 559,966
Net operating income	16,569	40,834	23,292	1,152	11,648	35,804	20,783	5,770	81,847	74,005	73,843
Net income (loss)	$ 11,394	$ 33,650	$ 17,170	$ (5,476)	$ 5,666	$ 29,151	$ 13,510	$ (1,073)
Earnings per share-diluted	$ 0.24	$ 0.70	$ 0.36	$ (0.11)	$ 0.12	$ 0.61	$ 0.28	$ (0.03)	$ 1.19	$ 1.02	$ 1.17
Dividends paid per common share (in dollars per share)	$ 0.1625	$ 0.1625	$ 0.1625	$ 0.1625	$ 0.1600	$ 0.1600	$ 0.1600	$ 0.1600	$ 0.65	$ 0.64	$ 0.63
Maximum
Common stock market price range:
Common stock market price (in dollars per share)	$ 26.37	$ 24.78	$ 24.37	$ 24.60	$ 23.43	$ 22.34	$ 20.84	$ 21.22	$ 26.37	$ 23.43
Minimum
Common stock market price range:
Common stock market price (in dollars per share)	$ 21.98	$ 22.41	$ 20.33	$ 21.63	$ 19.65	$ 18.87	$ 18.54	$ 18.42	$ 21.98	$ 19.65