Note 9 - Retirement Plans - Pension Plan Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined benefit plan, fair value of plan assets	$ 83,415	$ 88,248	$ 80,929
Fair Value, Inputs, Level 1 [Member]
Defined benefit plan, fair value of plan assets	66,957	71,375
Fair Value, Inputs, Level 2 [Member]
Defined benefit plan, fair value of plan assets	16,458	16,873
Fair Value, Inputs, Level 3 [Member]
Defined benefit plan, fair value of plan assets	0	0
Insurance General Account [Member]
Defined benefit plan, fair value of plan assets	16,458	16,873
Insurance General Account [Member] | Fair Value, Inputs, Level 1 [Member]
Defined benefit plan, fair value of plan assets	0	0
Insurance General Account [Member] | Fair Value, Inputs, Level 2 [Member]
Defined benefit plan, fair value of plan assets	16,458	16,873
Insurance General Account [Member] | Fair Value, Inputs, Level 3 [Member]
Defined benefit plan, fair value of plan assets	0	0
Cash and Cash Equivalents [Member]
Defined benefit plan, fair value of plan assets	721	3,229
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Defined benefit plan, fair value of plan assets	721	3,229
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member]
Defined benefit plan, fair value of plan assets	0	0
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member]
Defined benefit plan, fair value of plan assets	0	0
Equity Securities [Member]
Defined benefit plan, fair value of plan assets	39,047	37,536
Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined benefit plan, fair value of plan assets	39,047	37,536
Equity Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined benefit plan, fair value of plan assets	0	0
Equity Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined benefit plan, fair value of plan assets	0	0
Fixed Income Securities [Member]
Defined benefit plan, fair value of plan assets	25,878	27,146
Fixed Income Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined benefit plan, fair value of plan assets	25,878	27,146
Fixed Income Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined benefit plan, fair value of plan assets	0	0
Fixed Income Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined benefit plan, fair value of plan assets	0	0
Real Estate [Member]
Defined benefit plan, fair value of plan assets	1,311	3,464
Real Estate [Member] | Fair Value, Inputs, Level 1 [Member]
Defined benefit plan, fair value of plan assets	1,311	3,464
Real Estate [Member] | Fair Value, Inputs, Level 2 [Member]
Defined benefit plan, fair value of plan assets	0	0
Real Estate [Member] | Fair Value, Inputs, Level 3 [Member]
Defined benefit plan, fair value of plan assets	$ 0	$ 0