Note 8 - Employee Benefit Plans - Fair Value Measurements (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Fair value, plan assets		$ 23,428	$ 17,038
Fair Value, Inputs, Level 1 [Member]
Fair value, plan assets		16,414	10,484
Fair Value, Inputs, Level 2 [Member]
Fair value, plan assets		7,014	6,554
Fair Value, Inputs, Level 3 [Member]
Fair value, plan assets		0	0
Cash [Member]
Fair value, plan assets		4,454	207
Cash [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value, plan assets		4,454	207
Cash [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value, plan assets		0	0
Cash [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value, plan assets		0	0
Equity Securities US Companies [Member]
Fair value, plan assets		9,892	9,195
Equity Securities US Companies [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value, plan assets		9,892	9,195
Equity Securities US Companies [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value, plan assets		0	0
Equity Securities US Companies [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value, plan assets		0	0
Equity Securities International Companies [Member]
Fair value, plan assets		106	116
Equity Securities International Companies [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value, plan assets		106	116
Equity Securities International Companies [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value, plan assets		0	0
Equity Securities International Companies [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value, plan assets		0	0
Equity Funds [Member]
Fair value, plan assets	[1]	1,962	966
Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value, plan assets	[1]	1,962	966
Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value, plan assets	[1]	0	0
Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value, plan assets	[1]	0	0
US Government Corporations and Agencies Securities [Member]
Fair value, plan assets		51	53
US Government Corporations and Agencies Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value, plan assets		0	0
US Government Corporations and Agencies Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value, plan assets		51	53
US Government Corporations and Agencies Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value, plan assets		0	0
US States and Political Subdivisions Debt Securities [Member]
Fair value, plan assets		231	230
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value, plan assets		0	0
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value, plan assets		231	230
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value, plan assets		0	0
Corporate Debt Securities [Member] | External Credit Rating, Investment Grade [Member]
Fair value, plan assets	[2]	6,732	6,271
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member] | External Credit Rating, Investment Grade [Member]
Fair value, plan assets	[2]	0	0
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | External Credit Rating, Investment Grade [Member]
Fair value, plan assets	[2]	6,732	6,271
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | External Credit Rating, Investment Grade [Member]
Fair value, plan assets	[2]	$ 0	$ 0

[1] This category comprises actively managed equity funds invested in large-cap and mid-cap U.S. companies.

[2] This category represents investment grade bonds of U.S. issuers from diverse industries.