SELECTED QUARTERLY INFORMATION (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
SELECTED QUARTERLY INFORMATION (UNAUDITED)
Operating revenues	$ 129,119	$ 159,202	$ 131,632	$ 137,144	$ 132,765	$ 164,284	$ 129,093	$ 150,728	$ 557,097	$ 576,870	$ 541,276
Operating income	19,367	35,282	20,762	20,810	19,502	36,450	19,134	21,848	96,221	96,934	80,495
Net income	$ 9,627	$ 25,542	$ 10,708	$ 9,804	$ 8,690	$ 25,184	$ 9,175	$ 11,922	$ 55,681	$ 54,971	$ 47,396
Basic Earning Per Share (in dollars per share)	$ 0.23	$ 0.60	$ 0.25	$ 0.23	$ 0.21	$ 0.60	$ 0.22	$ 0.29
Diluted Earnings Per Share (in dollars per share)	$ 0.23	$ 0.60	$ 0.25	$ 0.23	$ 0.21	$ 0.60	$ 0.22	$ 0.29