Note 8 - Employee Benefit Plans (Details Textual) - USD ($)		1 Months Ended		12 Months Ended
		Jul. 31, 2017	Apr. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase				2.00%	2.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets				6.62%	7.20%
Long-term Investments				$ 7,561,000	$ 5,002,000
Defined Benefit Plan Assumed Long Term Rate Basis Spread				1.00%
Defined Benefit Plan, Fair Value of Plan Assets				$ 99,922,000	87,357,000	$ 73,321,000
Defined Benefit Plan, Plan Assets, Contributions by Employer				$ 7,227,000	9,092,000
Defined Contribution Plan, Employer Matching Contribution, Percent of Match				55.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay				5.00%
Defined Contribution Plan, Employer Discretionary Contribution Amount				$ 114,000	442,000	391,000
Capital Accumulation 401(K) Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer				$ 3,500,000	3,500,000	$ 3,400,000
Defined Contribution Plan, Employer Matching Contribution, Percent of Match				100.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay				6.00%
Minimum [Member]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent				2.00%
Minimum [Member] | Capital Accumulation 401(K) Plan [Member]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent				1.00%
Maximum [Member]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent				50.00%
Maximum [Member] | Capital Accumulation 401(K) Plan [Member]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent				50.00%
Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer		$ 5,700,000	$ 1,200,000
Pension Plan [Member] | Common Stock [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year				$ 3,800,000
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year				500,000
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Fair Value of Plan Assets				17,100,000	14,000,000
Company Stock [Member]
Long-term Investments				4,800,000	6,300,000
Real Estate [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Fair Value of Plan Assets				14,200,000	11,400,000
Collective Investment Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Fair Value of Plan Assets				$ 2,900,000	$ 2,600,000
Assumed for 2016 Through 2018 [Member]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	[1]				0.00%
Assumed for after 2018 [Member]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	[1]			2.00%	2.00%

[1] 0% was assumed for years 2016 through 2018 and 2% was assumed thereafter.