Note 4 - Operating Assets and Liabilities - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Beginning balance	$ 1,252	$ 1,201
Additions - charged to costs and expenses	643	2,118
Deductions - write-offs	(2,451)	(2,271)
Recoveries collected	1,930	1,949
Ending balance	$ 1,374	$ 2,997