QUARTERLY FINANCIAL DATA (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
Operating revenue	$ 103,014	$ 169,254	$ 131,397	$ 98,149	$ 105,457	$ 146,349	$ 118,321	$ 90,272	$ 501,814	$ 460,399	$ 449,372
Net operating income	7,586	30,078	19,555	9,948	11,906	25,865	16,284	7,758	67,167	61,813	58,119
Net income	$ 1,868	$ 20,935	$ 12,190	$ 2,719	$ 4,871	$ 20,386	$ 10,381	$ 2,018	$ 37,712	$ 37,656	$ 40,554
Diluted earnings per share (in dollars per share)	$ 0.04	$ 0.50	$ 0.29	$ 0.07	$ 0.11	$ 0.49	$ 0.25	$ 0.05	$ 0.90	$ 0.90	$ 0.98
Common stock market price range:
Dividends paid per common share (in dollars per share)	$ 0.15375	$ 0.15375	$ 0.15375	$ 0.15375	$ 0.14875	$ 0.14875	$ 0.14875	$ 0.14875
High
Common stock market price range:
Common stock market price (in dollars per share)	$ 19.20	$ 19.37	$ 19.16	$ 19.18	$ 19.25	$ 18.87	$ 19.85	$ 19.05	$ 19.20	$ 19.25
Low
Common stock market price range:
Common stock market price (in dollars per share)	$ 16.81	$ 16.65	$ 18.06	$ 17.28	$ 18.01	$ 16.93	$ 16.91	$ 17.63	$ 16.81	$ 18.01