QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data [Line Items]
Operating revenue	$ 138,426	$ 183,543	$ 144,414	$ 121,985	$ 137,384	$ 191,184	$ 158,416	$ 110,515	$ 588,368	$ 597,499	$ 584,103
Net operating income	14,962	32,201	16,493	7,497	16,569	40,834	23,292	1,152	$ 71,153	$ 81,847	$ 74,005
Net income (loss)	$ 8,477	$ 25,120	$ 9,845	$ 1,575	$ 11,394	$ 33,650	$ 17,170	$ (5,476)
Earnings per share—diluted (in dollars per share)	$ 0.18	$ 0.52	$ 0.21	$ 0.03	$ 0.24	$ 0.70	$ 0.36	$ (0.11)	$ 0.94	$ 1.19	$ 1.02
Dividends paid per common share (in dollars per share)	0.1675	0.1675	0.1675	0.1675	0.1625	0.1625	0.1625	0.1625	0.670	0.650	$ 0.640
Maximum
Common stock market price range:
Common stock market price (in dollars per share)	24.35	24.36	25.30	25.99	26.37	24.78	24.37	24.60	24.35	26.37
Minimum
Common stock market price range:
Common stock market price (in dollars per share)	$ 21.01	$ 19.55	$ 22.58	$ 23.63	$ 21.98	$ 22.41	$ 20.33	$ 21.63	$ 21.01	$ 21.98