Note 13 - Income Taxes - Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Federal tax provision at statutory rate	$ 99,233	$ 91,947	$ 73,673
State taxes, net of federal income tax benefit	10,784	9,357	6,526
Equity investment basis difference	9,470	11,048	1,422
Non-deductible items	1,436	882	1,766
Permanent differences related to employee stock purchase program	139	156	68
Net change in valuation allowance	(5,133)	(3,303)	(4,121)
General business credits	(27,950)	(29,093)	(4,002)
Other	(1,514)	(1,289)	(377)
Income tax provision	$ 86,465	$ 79,705	$ 74,955