Note 19 - Quarterly Financial Data (Unaudited) - Quarterly Results of Operations (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
Operating revenue	$ 217,185	$ 214,973	$ 212,090	$ 199,023	$ 203,961	$ 199,649	$ 197,024	$ 186,960	$ 843,271	$ 787,594	$ 698,120
Operating income	18,787	20,034	19,944	17,733	19,538	18,993	18,223	13,594	76,498	70,348	56,862
Net income	$ 15,748	$ 16,587	$ 15,190	$ 13,546	$ 15,737	$ 15,257	$ 13,702	$ 10,331	$ 61,071	$ 55,027	$ 90,284
Basic earnings per common share (in dollars per share)	$ 0.29	$ 0.30	$ 0.28	$ 0.25	$ 0.29	$ 0.28	$ 0.25	$ 0.19	$ 1.12	$ 1.01	$ 1.66
Diluted earnings per common share (in dollars per share)	$ 0.29	$ 0.30	$ 0.28	$ 0.25	$ 0.29	$ 0.28	$ 0.25	$ 0.19	$ 1.11	$ 1.00	$ 1.65