Note 9 - Stock-based Compensation - Phantom Stock Plan (Details) - Phantom Share Units (PSUs) [Member] - Phantom Stock Plan [Member]	3 Months Ended
	Mar. 31, 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	8,168
Granted (in dollars per share) | $ / shares	$ 22.15
Distributions (in shares) | shares	(1,004)
Distributions (in dollars per share) | $ / shares	$ 22.01
Outstanding, ending balance (in shares) | shares	106,477
Outstanding, ending (in dollars per share) | $ / shares	$ 21.93
Vested at March 31, 2019 (in shares) | shares	105,935
Vested at March 31, 2019 (in dollars per share) | $ / shares	$ 21.93