Note 10 - Pension Plan and Other Postretirement Benefits - Pension Fund Assets Measured at Fair Value (Details) - Pension Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value of Plan Assets	$ 285,319	$ 254,346	$ 233,639
Fair Value, Inputs, Level 1 [Member]
Fair Value of Plan Assets	273,999	234,303
Fair Value, Inputs, Level 1 [Member] | Large Capitalization Equity Securities [Member]
Fair Value of Plan Assets	66,946	54,483
Fair Value, Inputs, Level 1 [Member] | International Equity Securities [Member]
Fair Value of Plan Assets	51,636	55,916
Fair Value, Inputs, Level 1 [Member] | Small and Mid Capitalization Equity Securities [Member]
Fair Value of Plan Assets	24,848	23,011
Fair Value, Inputs, Level 1 [Member] | Emerging Markets Equity Funds [Member]
Fair Value of Plan Assets	15,824
Fair Value, Inputs, Level 1 [Member] | SEI Dynamic Asset Allocation Fund [Member]
Fair Value of Plan Assets	14,371	13,622
Fair Value, Inputs, Level 1 [Member] | Fixed Income Securities [Member]
Fair Value of Plan Assets	100,373	87,268
Fair Value, Inputs, Level 1 [Member] | Cash Management Money Market Fund [Member]
Fair Value of Plan Assets	$ 1	$ 3