Note 4 - Operating Assets and Liabilities - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Beginning balance	$ 1,201	$ 2,045
Additions - charged to costs and expenses	2,118	1,570
Deductions	(2,271)	(4,754)
Recoveries of amounts previously written off	1,949	2,090
Ending balance	$ 2,997	$ 951