Note 7 - Stock-based Compensation - Phantom Stock Plan (Details) - Phantom Share Units (PSUs) [Member] - Phantom Stock Plan [Member]	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Outstanding, beginning balance (in shares) | shares	89,339
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 29.39
Granted (in shares) | shares	7,889
Granted (in dollars per share) | $ / shares	$ 27.42
Forfeited (in shares) | shares	(10)
Forfeited (in dollars per share) | $ / shares	$ 28.95
Distributions (in shares) | shares	(8,471)
Distributions (in dollars per share) | $ / shares	$ 28.69
Outstanding, ending balance (in shares) | shares	88,747
Outstanding, ending (in dollars per share) | $ / shares	$ 29.72
Vested at September 30, 2017 (in shares) | shares	88,431
Vested at September 30, 2017 (in dollars per share) | $ / shares	$ 29.72