Note 15 - Net Income Per Common Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended												12 Months Ended
		Dec. 31, 2019	[1]	Sep. 30, 2019	[2]	Jun. 30, 2019	[2]	Mar. 31, 2019	[3]	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income		$ 53,613		$ 49,835		$ 36,395		$ 38,739		$ 42,008	$ 38,314	$ 43,785	$ 40,653	$ 178,582	$ 164,760	$ 235,171
Basic (in shares)		5,685,840		5,682,167		5,673,669		5,674,120		5,674,067	5,674,224	5,687,095	5,702,539	5,678,990	5,684,375	5,680,073
Effect of dilutive equity-based awards (1) (in shares)	[4]													58,866	41,588	66,964
Diluted (in shares)		5,751,970		5,741,666		5,730,238		5,716,585		5,723,528	5,717,575	5,722,869	5,742,648	5,737,856	5,725,963	5,747,037
Basic (in dollars per share)		$ 9.43		$ 8.77		$ 6.41		$ 6.83		$ 7.40	$ 6.75	$ 7.70	$ 7.13	$ 31.45	$ 28.98	$ 41.40
Diluted (in dollars per share)		$ 9.32		$ 8.68		$ 6.35		$ 6.78		$ 7.34	$ 6.70	$ 7.65	$ 7.08	$ 31.12	$ 28.77	$ 40.92

[1] Includes Clearwave and Fidelity operations.

[2] Includes Clearwave operations.

[3] Includes Clearwave operations beginning January 8, 2019.

[4] Equity-based awards whose impact is considered to be anti-dilutive under the treasury stock method were excluded from the diluted net income per common share calculation. The excluded number of anti-dilutive equity-based awards totaled 409, 1,811 and 2,600 for 2019, 2018 and 2017, respectively.