Note 14 - Income Taxes - Summary of Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Tax expense computed at federal statutory rate	$ 55,409	$ 80,406	$ 53,123
State income tax expense, net of federal benefit	9,661	6,432	4,553
Other permanent differences	(31)	(748)	(647)
Domestic manufacturing deduction		(5,387)	(5,563)
Limitation on executive compensation	2,912
Rate change effect of tax method changes	(5,661)
Tax expense (benefit) related to an increase (decrease) in unrecognized tax benefits	4,680	75	75
Expiration of stock-based compensation	415	2,832
Federal energy credits	(12,446)		(3,428)
Rate changes	(78)	10,018
Change in valuation allowance	(885)	(8,978)	253
Other	(902)	3,247	204
Provision for income taxes	$ 53,074	$ 87,897	$ 48,570
Effective tax (benefit) rate	20.10%	38.30%	32.00%