Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information (Unaudited) [Abstract]
OPERATING REVENUE	$ 147,003	$ 151,279	$ 155,826	$ 156,695	$ 165,703	$ 182,114	$ 129,616	$ 129,903	$ 610,803	$ 607,336	$ 612,937
Operating Income (Loss)	29,560	25,132	22,147	12,948	9,869	12,999	21,313	19,363	89,787	63,544	85,568
Income before income taxes	30,339	25,718	22,570	13,290	10,048	13,062	21,737	19,651	91,917	64,498	86,049
Net income	$ 22,440	$ 19,056	$ 17,803	$ 13,378	$ 38,605	$ 7,916	$ 14,616	$ 14,036	$ 72,677	$ 75,173	$ 56,386
Basic EPS	$ 0.27	$ 0.23	$ 0.22	$ 0.16	$ 0.45	$ 0.10	$ 0.18	$ 0.17	$ 0.88	$ 0.90	$ 0.68
Diluted EPS	$ 0.27	$ 0.23	$ 0.22	$ 0.16	$ 0.45	$ 0.09	$ 0.18	$ 0.17	$ 0.88	$ 0.90	$ 0.68