SELECTED QUARTERLY INFORMATION (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
Operating revenues	$ 136,758	$ 169,714	$ 134,557	$ 164,544	$ 151,363	$ 171,512	$ 149,782	$ 179,673	$ 605,573	$ 652,330	$ 594,330
Operating income	19,757	35,783	16,047	24,713	19,300	31,709	19,502	29,488	96,301	99,999	99,663
Net income	$ 9,905	$ 25,285	$ 6,770	$ 14,637	$ 11,112	$ 23,892	$ 11,194	$ 20,905	$ 56,597	$ 67,103	$ 63,445
Basic Earnings Per Share	$ 0.23	$ 0.58	$ 0.16	$ 0.34
Diluted Earnings Per Share	$ 0.23	$ 0.58	$ 0.15	$ 0.34
Basic and Diluted Earnings Per Share					$ 0.26	$ 0.55	$ 0.26	$ 0.48
SPP IM
Operating revenues	$ 2,900	$ 4,000	$ 3,400	$ 4,700	$ 7,500	$ 11,500	$ 16,500	$ 6,200