Quarterly Financial Data (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 REVENUES	$ 210,386	$ 217,210	$ 209,589	$ 191,911	$ 169,671	$ 156,492	$ 158,453	$ 145,579	$ 829,096	$ 630,195	$ 544,866
INCOME FROM OPERATIONS	45,388	56,023	51,165	39,203	23,919	38,512	37,873	30,106	191,779	130,410	99,322
Income from continuing operations, net of tax	33,274	32,952	31,145	22,122	19,952	22,050	20,691	15,976	119,493	78,669	60,075
(Loss)/income from discontinued operations, net of tax	1	(10)	0	0	(82)	2,745	(56)	(16)	(9)	2,591	1,398
Net income	$ 33,275	$ 32,942	$ 31,145	$ 22,122	$ 19,870	$ 24,795	$ 20,635	$ 15,960	$ 119,484	$ 81,260	$ 61,473
Diluted earnings per common share from continuing operations	$ 0.77	$ 0.77	$ 0.73	$ 0.52	$ 0.47	$ 0.53	$ 0.50	$ 0.39	$ 2.79	$ 1.88	$ 1.54
Diluted earnings per common share from discontinued operations	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.07	$ 0.00	$ 0.00	$ 0.00	$ 0.06	$ 0.04
Diluted earnings per common share	$ 0.77	$ 0.77	$ 0.73	$ 0.52	$ 0.47	$ 0.59	$ 0.49	$ 0.39	$ 2.79	$ 1.94	$ 1.57