QUARTERLY FINANCIAL DATA (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
Operating revenue	$ 121,530	$ 178,135	$ 143,552	$ 116,749	$ 103,014	$ 169,254	$ 131,397	$ 98,149	$ 559,966	$ 501,814	$ 460,399
Net operating income	11,253	36,361	19,622	6,607	7,586	30,078	19,555	9,948	73,843	67,167	61,813
Net income	$ 5,006	$ 29,772	$ 12,965	$ 1,085	$ 1,868	$ 20,935	$ 12,190	$ 2,719
Diluted earnings per share (in dollars per share)	$ 0.12	$ 0.71	$ 0.31	$ 0.03	$ 0.04	$ 0.50	$ 0.29	$ 0.07	$ 1.17	$ 0.90	$ 0.90
Common stock market price range:
Dividends paid per common share (in dollars per share)	$ 0.1575	$ 0.1575	$ 0.1575	$ 0.1575	$ 0.15375	$ 0.15375	$ 0.15375	$ 0.15375	$ 0.630	$ 0.615	$ 0.595
High
Common stock market price range:
Common stock market price (in dollars per share)	$ 18.90	$ 19.05	$ 18.60	$ 19.25	$ 19.20	$ 19.37	$ 19.16	$ 19.18	$ 18.90	$ 19.20
Low
Common stock market price range:
Common stock market price (in dollars per share)	$ 16.84	$ 17.93	$ 17.14	$ 17.67	$ 16.81	$ 16.65	$ 18.06	$ 17.28	$ 16.84	$ 16.81