Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information (Unaudited) [Abstract]
OPERATING REVENUE	$ 167,227	$ 147,908	$ 142,144	$ 139,536	$ 147,003	$ 151,279	$ 155,826	$ 156,695	$ 596,815	$ 610,803	$ 607,336
Operating Income (Loss)	17,663	26,739	29,030	20,843	29,560	25,132	22,147	12,948	94,275	89,787	63,544
Income before income taxes	17,516	27,415	30,259	21,988	30,339	25,718	22,570	13,290	97,178	91,917	64,498
Net income	$ 12,787	$ 20,501	$ 22,361	$ 17,318	$ 22,440	$ 19,056	$ 17,803	$ 13,378	$ 72,967	$ 72,677	$ 75,173
Earnings Per Share Basic	$ 0.16	$ 0.25	$ 0.27	$ 0.21	$ 0.27	$ 0.23	$ 0.22	$ 0.16	$ 0.89	$ 0.88	$ 0.9
Earnings Per Share Diluted	$ 0.16	$ 0.25	$ 0.27	$ 0.21	$ 0.27	$ 0.23	$ 0.22	$ 0.16	$ 0.89	$ 0.88	$ 0.9