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, 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
Operating revenue		$ 203,961	$ 199,649	$ 197,024	$ 186,960	$ 182,771	$ 170,679	$ 171,511	$ 173,159	$ 787,594	$ 698,120	$ 671,144
Operating income		19,538	18,993	18,223	13,594	14,333	13,022	15,569	13,938	70,348	56,862	58,303
Net income		$ 15,737	$ 15,257	$ 13,702	$ 10,331	$ 65,074	$ 7,855	$ 9,141	$ 8,214	$ 55,027	$ 90,284	$ 33,464
Basic earnings per common share (in dollars per share)		$ 0.29	$ 0.28	$ 0.25	$ 0.19	$ 1.19	$ 0.14	$ 0.17	$ 0.15	$ 1.01	$ 1.66	$ 0.62
Diluted earnings per common share (in dollars per share)		$ 0.29	$ 0.28	$ 0.25	$ 0.19	$ 1.18	$ 0.14	$ 0.17	$ 0.15	$ 1	$ 1.65	$ 0.61
Net income – excluding deferred income taxes benefit(1)	[1]					$ 8,609	$ 7,855	$ 9,141	$ 8,214
Basic earnings per common share – excluding deferred income taxes benefit(1) (in dollars per share)	[1]					$ 0.16	$ 0.14	$ 0.17	$ 0.15
Diluted earnings per common share – excluding deferred income taxes benefit(1) (in dollars per share)	[1]					$ 0.16	$ 0.14	$ 0.17	$ 0.15

[1] The amounts are presented for comparative purposes excluding the $56.5 million deferred income taxes benefit recorded to recognize the impact on our federal net deferred tax liability of the reduction of the federal corporate statutory income tax rate from 35% to 21% related to the Tax Cuts and Jobs Act of 2017.