Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information (Unaudited) [Abstract]
OPERATING REVENUE	$ 155,789	$ 162,282	$ 160,873	$ 166,318	$ 167,227	$ 147,908	$ 142,144	$ 139,536	$ 645,262	$ 596,815	$ 610,803
Operating Income (Loss)	23,765	27,339	24,989	17,326	17,663	26,739	29,030	20,843	93,419	94,275	89,787
Income before income taxes	23,902	27,510	25,146	17,703	17,516	27,415	30,259	21,988	94,261	97,178	91,917
Net income	$ 17,672	$ 20,714	$ 19,182	$ 13,238	$ 12,787	$ 20,501	$ 22,361	$ 17,318	$ 70,806	$ 72,967	$ 72,677
Earnings Per Share Basic	$ 0.22	$ 0.25	$ 0.24	$ 0.16	$ 0.16	$ 0.25	$ 0.27	$ 0.21	$ 0.87	$ 0.89	$ 0.88
Earnings Per Share Diluted	$ 0.22	$ 0.25	$ 0.24	$ 0.16	$ 0.16	$ 0.25	$ 0.27	$ 0.21	$ 0.87	$ 0.89	$ 0.88