SELECTED QUARTERLY INFORMATION (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
SELECTED QUARTERLY INFORMATION (UNAUDITED)
Operating revenues	$ 132,765	$ 164,284	$ 129,093	$ 150,728	$ 132,815	$ 154,086	$ 114,482	$ 139,893	$ 576,870	$ 541,276	$ 497,168
Operating income	19,502	36,450	19,134	21,848	18,265	31,873	14,279	16,078	96,934	80,495	74,495
Net income	$ 8,690	$ 25,184	$ 9,175	$ 11,922	$ 8,460	$ 22,981	$ 7,369	$ 8,586	$ 54,971	$ 47,396	$ 41,296
Basic Earning Per Share (in dollars per share)	$ 0.21	$ 0.60	$ 0.22	$ 0.29	$ 0.20	$ 0.56	$ 0.18	$ 0.22
Diluted Earnings Per Share (in dollars per share)	$ 0.21	$ 0.60	$ 0.22	$ 0.29	$ 0.20	$ 0.55	$ 0.18	$ 0.22