Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information (Unaudited) [Abstract]
OPERATING REVENUE	$ 140,044	$ 149,316	$ 160,791	$ 162,786	$ 174,605	$ 182,533	$ 191,684	$ 187,523	$ 612,937	$ 736,345	$ 871,355
Operating Income (Loss)	20,898	19,913	24,507	20,250	27,719	24,861	35,739	28,261	85,568	116,580	131,868
Income before income taxes	21,071	20,037	24,616	20,325	27,772	24,926	35,800	28,273	86,049	116,771	131,617
Net income	$ 13,114	$ 12,527	$ 16,368	$ 14,377	$ 17,015	$ 15,113	$ 23,316	$ 17,612	$ 56,386	$ 73,056	$ 84,834
Basic EPS	$ 0.16	$ 0.15	$ 0.20	$ 0.17	$ 0.20	$ 0.17	$ 0.27	$ 0.20	$ 0.68	$ 0.84	$ 0.97
Diluted EPS	$ 0.16	$ 0.15	$ 0.20	$ 0.17	$ 0.20	$ 0.17	$ 0.27	$ 0.20	$ 0.68	$ 0.84	$ 0.96