Note 9 - Stock-based Compensation - Phantom Stock Plan (Details) - Phantom Share Units (PSUs) [Member] - Phantom Stock Plan [Member]	6 Months Ended
	Jun. 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	9,175
Granted (in dollars per share) | $ / shares	$ 22.15
Distributions (in shares) | shares	(1,012)
Distributions (in dollars per share) | $ / shares	$ 22.00
Outstanding, ending balance (in shares) | shares	107,476
Outstanding, ending (in dollars per share) | $ / shares	$ 22.20
Vested at June 30, 2019 (in shares) | shares	106,929
Vested at June 30, 2019 (in dollars per share) | $ / shares	$ 22.20