Note 2 - Accumulated Other Comprehensive Income (Details) - Amounts Reclassified out of Accumulated Other Comprehensive Income by Component - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	$ (1,080)	$ (1,439)	$ (1,369)	$ (1,405)	$ (1,737)	$ (1,283)	$ (1,959)	$ (1,120)	$ (5,293)	$ (6,099)	$ (4,435)
Net of tax	3,584	3,832	3,332	3,849	3,781	3,657	4,261	2,833	14,597	14,532	11,495
Amortization related to postretirement benefits
	$ 4,664	$ 5,271	$ 4,871	$ 5,479	$ 5,895	$ 5,390	$ 6,783	$ 4,490	20,285	22,558	$ 17,881
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									45	(154)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains and losses on available for sale investment securities									161	(97)
Income tax expense									(56)	34
Net of tax									105	(63)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense									33	49
Net of tax									(60)	(91)
Amortization related to postretirement benefits
Prior service costs									(51)	(207)
Actuarial (losses) gains									(42)	67
									$ (93)	$ (140)