Quarterly Financial Information (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
Quarterly Financial Information (Unaudited) [Abstract]
Operating Revenue	$ 136,192	$ 135,010	$ 139,710	$ 134,833	$ 131,210	$ 132,529	$ 137,192	$ 127,692	$ 545,745	$ 528,623	$ 499,516
Operating Income (Loss)	24,708	19,667	26,748	23,778	26,865	25,132	32,687	21,873	94,901	106,557	91,449
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	24,882	19,858	26,915	23,920	27,019	25,306	32,895	22,110	95,575	107,330	92,873
Net income	$ 14,292	$ 12,434	$ 18,227	$ 16,588	$ 17,122	$ 15,399	$ 22,532	$ 14,879	$ 61,541	$ 69,932	$ 62,216
Basic EPS	$ 0.17	$ 0.15	$ 0.21	$ 0.19	$ 0.20	$ 0.17	$ 0.25	$ 0.16	$ 0.72	$ 0.78	$ 0.69
Diluted EPS	$ 0.17	$ 0.14	$ 0.21	$ 0.19	$ 0.20	$ 0.17	$ 0.25	$ 0.16	$ 0.71	$ 0.78	$ 0.69