Note 8 - Retirement Plans - Pension Plan Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined benefit plan, fair value of plan assets	$ 80,929	$ 69,246	$ 66,813
Fair Value, Inputs, Level 2 [Member]
Defined benefit plan, fair value of plan assets	18,357	17,918
Fair Value, Inputs, Level 1 [Member]
Defined benefit plan, fair value of plan assets	62,572	51,328
Insurance General Account [Member]
Defined benefit plan, fair value of plan assets	18,357	17,918
Insurance General Account [Member] | Fair Value, Inputs, Level 2 [Member]
Defined benefit plan, fair value of plan assets	18,357	17,918
Cash and Cash Equivalents [Member]
Defined benefit plan, fair value of plan assets	5,089	1,273
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Defined benefit plan, fair value of plan assets	5,089	1,273
Equity Securities [Member]
Defined benefit plan, fair value of plan assets	31,963	37,621
Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined benefit plan, fair value of plan assets	31,963	37,621
Fixed Income Securities [Member]
Defined benefit plan, fair value of plan assets	22,544	9,924
Fixed Income Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined benefit plan, fair value of plan assets	22,544	9,924
Real Estate [Member]
Defined benefit plan, fair value of plan assets	2,976	2,510
Real Estate [Member] | Fair Value, Inputs, Level 1 [Member]
Defined benefit plan, fair value of plan assets	$ 2,976	$ 2,510