Note 15 - Income Taxes - Summary of Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Tax expense computed at federal statutory rate	$ 64,048	$ 55,409	$ 80,406
State income tax expense, net of federal benefit	9,810	9,661	6,432
Other permanent differences	265	(31)	(748)
Domestic manufacturing deduction	0	0	(5,387)
Limitation on executive compensation	4,463	2,912	0
Rate change effect of tax method changes	0	(5,661)	0
Tax expense (benefit) related to an increase (decrease) in unrecognized tax benefits	(1,571)	4,680	75
Stock based compensation (windfall)/shortfall	(2,828)	415	2,832
Federal energy credits	(7,649)	(12,446)	0
Rate changes	190	(78)	10,018
Change in valuation allowance	121	(885)	(8,978)
Other	(172)	(902)	3,247
Provision for income taxes	$ 66,677	$ 53,074	$ 87,897
Effective tax rate	21.90%	20.10%	38.30%