QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data [Line Items]
Operating revenue	$ 150,930	$ 184,268	$ 152,445	$ 121,727	$ 138,426	$ 183,543	$ 144,414	$ 121,985	$ 609,370	$ 588,368	$ 597,499
Net operating income	21,335	30,055	18,534	6,270	14,962	32,201	16,493	7,497	$ 76,194	$ 71,153	$ 81,847
Net income (loss)	$ 15,090	$ 22,875	$ 11,508	$ (798)	$ 8,477	$ 25,120	$ 9,845	$ 1,575
Earnings per share—diluted (in dollars per share)	$ 0.31	$ 0.48	$ 0.24	$ (0.02)	$ 0.18	$ 0.52	$ 0.21	$ 0.03	$ 1.01	$ 0.94	$ 1.19
Dividends paid per common share (in dollars per share)	0.1725	0.1725	0.1725	0.1725	0.1675	0.1675	0.1675	0.1675	0.690	0.670	$ 0.650
Maximum
Common stock market price range:
Common stock market price (in dollars per share)	36.85	35.62	34.95	27.33	24.35	24.36	25.30	25.99	36.85	24.35
Minimum
Common stock market price range:
Common stock market price (in dollars per share)	$ 29.25	$ 29.93	$ 26.22	$ 22.48	$ 21.01	$ 19.55	$ 22.58	$ 23.63	$ 29.25	$ 21.01