Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information (Unaudited) [Abstract]
OPERATING REVENUE	$ 174,605	$ 182,533	$ 191,684	$ 187,523	$ 202,997	$ 217,092	$ 226,785	$ 224,481	$ 736,345	$ 871,355	$ 582,257
Operating Income (Loss)	27,719	24,861	35,739	28,261	34,247	36,290	40,642	20,688	116,580	131,868	112,302
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	27,772	24,926	35,800	28,273	34,216	36,214	40,616	20,569	116,771	131,617	112,556
Net income	$ 17,015	$ 15,113	$ 23,316	$ 17,612	$ 21,544	$ 22,737	$ 26,472	$ 14,079	$ 73,056	$ 84,834	$ 70,582
Basic EPS	$ 0.20	$ 0.17	$ 0.27	$ 0.20	$ 0.25	$ 0.26	$ 0.30	$ 0.16	$ 0.84	$ 0.97	$ 0.83
Diluted EPS	$ 0.20	$ 0.17	$ 0.27	$ 0.20	$ 0.25	$ 0.26	$ 0.30	$ 0.16	$ 0.84	$ 0.96	$ 0.83