Quarterly Financial Data (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 REVENUES	$ 227,316	$ 222,745	$ 217,419	$ 207,436	$ 210,386	$ 217,210	$ 209,589	$ 191,911	$ 874,916	$ 829,096	$ 630,195
INCOME FROM OPERATIONS	33,660	52,875	62,473	41,314	45,388	56,023	51,165	39,203	190,322	191,779	130,410
Income from continuing operations, net of tax	13,487	(19,567)	36,387	22,244	33,274	32,952	31,145	22,122	52,551	119,493	78,669
(Loss)/income from discontinued operations, net of tax	(91)	0	(24)	(3)	1	(10)	0	0	(118)	(9)	2,591
Net income/(loss)	$ 9,021	$ (19,567)	$ 36,363	$ 22,241	$ 33,275	$ 32,942	$ 31,145	$ 22,122
Diluted earnings/(loss) per common share from continuing operations	$ 0.18	$ (0.47)	$ 0.84	$ 0.52	$ 0.77	$ 0.77	$ 0.73	$ 0.52	$ 1.02	$ 2.79	$ 1.88
Diluted (loss)/earnings per common share from discontinued operations	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.06
Diluted earnings per common share	$ 0.18	$ (0.47)	$ 0.84	$ 0.52	$ 0.77	$ 0.77	$ 0.73	$ 0.52	$ 1.02	$ 2.79	$ 1.94