Note 9 - Stockholders' Equity - Dividend Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Per share amount (in dollars per share)	$ 0.34	$ 0.28	$ 0.22
Quarterly Dividends 1 [Member]
Record date	Apr. 05, 2019	Apr. 05, 2018
Per share amount (in dollars per share)	$ 1,672	$ 1,351
Quarterly Dividends 2 [Member]
Record date	Jun. 06, 2019	Jun. 07, 2018
Per share amount (in dollars per share)	$ 1,672	$ 1,353
Quarterly Dividends 3 [Member]
Record date	Sep. 05, 2019	Sep. 06, 2018
Per share amount (in dollars per share)	$ 1,691	$ 1,369
Quarterly Dividends 4 [Member]
Record date	Dec. 05, 2019	Dec. 06, 2018
Per share amount (in dollars per share)	$ 1,701	$ 1,372