Note 8 - Stock-based Compensation - Phantom Stock Plan (Details) - Phantom Share Units (PSUs) [Member] - Phantom Stock Plan [Member]	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Outstanding, beginning balance (in shares) | shares	89,180
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 27.50
Granted (in shares) | shares	8,946
Granted (in dollars per share) | $ / shares	$ 27.75
Forfeited (in shares) | shares
Forfeited (in dollars per share) | $ / shares
Distributions (in shares) | shares	(32)
Distributions (in dollars per share) | $ / shares	$ 27.18
Outstanding, ending balance (in shares) | shares	98,094
Outstanding, ending (in dollars per share) | $ / shares	$ 26.10
Vested at June 30, 2018 (in shares) | shares	97,515
Vested at June 30, 2018 (in dollars per share) | $ / shares	$ 26.10