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Note 18 - Quarterly Financial Data (Unaudited) - Quarterly Results of Operations (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating revenue $ 182,771 $ 170,679 $ 171,511 $ 173,159 $ 172,661 $ 170,464 $ 166,090 $ 161,929 $ 698,120 $ 671,144 $ 664,994
Operating income 14,333 13,022 15,569 13,938 14,533 14,869 14,776 14,125 56,862 58,303 61,063
Net income 65,074 7,855 9,141 8,214 $ 8,303 $ 8,437 $ 8,531 $ 8,193 $ 90,284 $ 33,464 $ 35,745
Net income – excluding deferred income taxes benefit(1) [1] $ 8,609 $ 7,855 $ 9,141 $ 8,214              
Basic earnings per common share (in dollars per share) $ 1.19 $ 0.14 $ 0.17 $ 0.15 $ 0.15 $ 0.16 $ 0.16 $ 0.15 $ 1.66 $ 0.62 $ 0.64
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 (in dollars per share) 1.18 0.14 0.17 0.15 $ 0.15 $ 0.15 $ 0.16 $ 0.15 $ 1.65 $ 0.61 $ 0.64
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 of the reduction of the federal corporate statutory income tax rate beginning on January 1, 2018 from 35% to 21% related to the Tax Cuts and Jobs Act of 2017.