Note 17 - Summary of Quarterly Operating Results (Unaudited) - Quarterly Financial Data (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
Revenue from Contract with Customer, Including Assessed Tax	$ 318,751		$ 284,991		$ 285,650		$ 278,605		$ 269,852	$ 268,268	$ 268,414	$ 265,761	$ 1,167,997	$ 1,072,295	$ 959,956
Total costs and expenses	230,244		204,858		211,536		210,908		200,588	204,949	197,746	201,100	857,546	804,383	723,617
Income from operations	88,507		80,133		74,114		67,697		69,264	63,319	70,668	64,661	310,451	267,912	236,339
Net Income (Loss) Attributable to Parent, Total	$ 53,613		$ 49,835		$ 36,395		$ 38,739		$ 42,008	$ 38,314	$ 43,785	$ 40,653	$ 178,582	$ 164,760	$ 235,171
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
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
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

[1] Includes Clearwave and Fidelity operations.

[2] Includes Clearwave operations.

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