Note 5 - Operating Assets and Liabilities - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Balance at beginning of period		$ 2,045	$ 1,876	$ 505
Charged to costs and expenses	[1]	6,500	5,101	4,925
Deductions		(7,344)	(4,932)	(3,554)
Balance at end of period		$ 1,201	$ 2,045	$ 1,876

[1] Additions for 2017 include a $1.1 million allowance for doubtful accounts assumed as part of the NewWave acquisition.