Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information (Unaudited) [Abstract]
OPERATING REVENUE	$ 202,997	$ 217,092	$ 226,785	$ 224,481	$ 183,348	$ 130,645	$ 133,992	$ 134,273	$ 871,355	$ 582,257	$ 545,745
Operating Income (Loss)	34,247	36,290	40,642	20,688	26,720	26,000	29,375	30,207	131,868	112,302	94,901
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	34,216	36,214	40,616	20,569	26,596	26,126	29,504	30,330	131,617	112,556	95,575
Net income	$ 21,544	$ 22,737	$ 26,472	$ 14,079	$ 15,842	$ 15,868	$ 19,138	$ 19,734	$ 84,834	$ 70,582	$ 61,541
Basic EPS	$ 0.25	$ 0.26	$ 0.30	$ 0.16	$ 0.18	$ 0.19	$ 0.23	$ 0.23	$ 0.97	$ 0.83	$ 0.72
Diluted EPS	$ 0.25	$ 0.26	$ 0.30	$ 0.16	$ 0.18	$ 0.19	$ 0.23	$ 0.23	$ 0.96	$ 0.83	$ 0.71