Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information (Unaudited) [Abstract]
Operating Revenue	$ 183,348	$ 130,645	$ 133,992	$ 134,273	$ 136,192	$ 135,010	$ 139,710	$ 134,833	$ 582,257	$ 545,745	$ 528,623
Operating Income (Loss)	26,720	26,000	29,375	30,207	24,708	19,667	26,748	23,778	112,302	94,901	106,557
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	26,596	26,126	29,504	30,330	24,882	19,858	26,915	23,920	112,556	95,575	107,330
Net income	$ 15,842	$ 15,868	$ 19,138	$ 19,734	$ 14,292	$ 12,434	$ 18,227	$ 16,588	$ 70,582	$ 61,541	$ 69,932
Basic EPS	$ 0.18	$ 0.19	$ 0.23	$ 0.23	$ 0.17	$ 0.15	$ 0.21	$ 0.19	$ 0.83	$ 0.72	$ 0.78
Diluted EPS	$ 0.18	$ 0.19	$ 0.23	$ 0.23	$ 0.17	$ 0.14	$ 0.21	$ 0.19	$ 0.83	$ 0.71	$ 0.78