Note 20 - Selected Quarterly Financial Data - Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Operating Revenues	$ 238,001	$ 231,281	$ 227,768	$ 229,588	$ 233,927	$ 225,386	$ 224,902	$ 222,407	$ 926,638	$ 906,622	$ 871,683
Income Before Non–Operating Income	17,687	10,590	14,641	17,624	20,223	13,761	15,706	17,436	60,542	67,126	68,011
Non–Operating Income	4,876	5,091	4,925	4,773	5,246	4,550	4,130	4,222
Net Income	13,863	11,110	11,866	13,699	14,976	12,567	12,358	13,242	50,538	53,143	53,369
Net Income Available to Common Stockholders	$ 13,863	$ 11,110	$ 11,866	$ 13,699	$ 14,644	$ 10,415	$ 10,191	$ 11,074	$ 50,538	$ 46,324	$ 44,699
Basic Earnings Per Share (in dollars per share)	$ 0.92	$ 0.73	$ 0.78	$ 0.92	$ 1.03	$ 0.75	$ 0.74	$ 0.80	$ 3.34	$ 3.34	$ 3.24
Diluted Earnings Per Share (in dollars per share)	$ 0.91	$ 0.73	$ 0.78	$ 0.91	$ 0.99	$ 0.72	$ 0.71	$ 0.77	$ 3.32	$ 3.20	$ 3.14
Preferred Dividends					$ 332	$ 2,152	$ 2,167	$ 2,168		$ 6,819	$ 8,670