Note 8 - Stock-based Compensation - Phantom Stock Plan (Details) - Phantom Share Units (PSUs) [Member] - Phantom Stock Plan [Member]	3 Months Ended
	Mar. 31, 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	6,743
Granted (in dollars per share) | $ / shares	$ 27.17
Forfeited (in shares) | shares	0
Forfeited (in dollars per share) | $ / shares	$ 0
Distributions (in shares) | shares	(198)
Distributions (in dollars per share) | $ / shares	$ 28.93
Outstanding, ending balance (in shares) | shares	95,884
Outstanding, ending (in dollars per share) | $ / shares	$ 26.87
Vested at March 31, 2017 (in shares) | shares	95,462
Vested at March 31, 2017 (in dollars per share) | $ / shares	$ 26.87