Note 3 - Net Income Per Share- Level 3 (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income-basic									$ 393,626	$ 672,309	$ 606,011
Net income-diluted									$ 393,626	$ 672,309	$ 606,011
Average common shares outstanding—basic									195,443	208,773	214,267
Dilutive effect of stock-based compensation arrangements									1,199	1,567	4,286
Adjusted average common shares outstanding—diluted	194,663	194,156	193,299	201,819	205,165	208,691	213,603	218,343	196,642	210,340	218,553
Net income per share—basic									$ 2.01	$ 3.22	$ 2.83
Net income per share—diluted	$ 0.76	$ 0.70	$ (0.15)	$ 0.70	$ 0.79	$ 0.83	$ 0.78	$ 0.78	$ 2.00	$ 3.20	$ 2.77
Stock Compensation Plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Ant-dilutive shares - Shares of common stock equivalents									865	221	337