Note 8 - Employee Benefit Plans (Details Textual) - USD ($)	1 Months Ended		12 Months Ended
	Jul. 31, 2016	Feb. 29, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase					2.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets				6.80%	7.20%
Long-term Investments				$ 5,002,000	$ 1,515,000
Defined Benefit Plan Assumed Long Term Rate Basis Spread				1.00%
Defined Benefit Plan, Fair Value of Plan Assets				$ 87,357,000	73,321,000		$ 74,551,000
Defined Benefit Plan, Contributions by Employer				$ 9,092,000	5,460,000
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 Benefit Plan Months of Services Required for Eligibility				180 days
Defined Contribution Plan, Employer Discretionary Contribution Amount				$ 442,000	391,000		292,000
Capital Accumulation 401(K) Plan [Member]
Defined Benefit Plan, Contributions by Employer				$ 3,500,000	3,400,000		$ 3,200,000
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, Contributions by Employer		$ 2,600,000
Pension Plan [Member] | Common Stock [Member]
Defined Benefit Plan, Contributions by Employer	$ 6,100,000
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Fair Value of Plan Assets				$ 14,000,000	19,000,000
Company Stock [Member]
Long-term Investments				6,300,000	4,300,000
Real Estate [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Fair Value of Plan Assets				11,400,000	13,200,000
Collective Investment Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Fair Value of Plan Assets				$ 2,600,000	$ 5,800,000
Assumed for 2016 Through 2018 [Member]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase				0.00%	0.00%	[1]
Assumed for after 2018 [Member]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase				2.00%	2.00%	[1]
Scenario, Forecast [Member] | Common Stock [Member]
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year			$ 4,000,000
Scenario, Forecast [Member] | Unfunded Supplemental Executive Retirement Plan [Member]
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year			$ 400,000

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