Note 9 - Stock-based Compensation - Phantom Stock Plan (Details) - Phantom Share Units (PSUs) [Member] - Phantom Stock Plan [Member]	9 Months Ended
	Sep. 30, 2019 $ / shares shares
Outstanding, beginning balance (in shares) | shares	99,313
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 21.53
Granted (in shares) | shares	10,278
Granted (in dollars per share) | $ / shares	$ 21.97
Distributions (in shares) | shares	(1,055)
Distributions (in dollars per share) | $ / shares	$ 21.91
Outstanding, ending balance (in shares) | shares	108,536
Outstanding, ending (in dollars per share) | $ / shares	$ 20.20
Vested at September 30, 2019 (in shares) | shares	108,228
Vested at September 30, 2019 (in dollars per share) | $ / shares	$ 20.20