Note 3 - Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended																12 Months Ended
		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income-basic																		$ 672,309		$ 606,011		$ 121,088
Net income —diluted																		$ 672,309		$ 606,011		$ 120,873
Average common shares outstanding—basic																		208,773		214,267		215,321
Dilutive effect of stock-based compensation arrangements	[1]																	1,567		4,286		4,305
Adjusted average common shares outstanding—diluted		205,165		208,691		213,603		218,343		217,883		217,902		217,830		219,883		210,340		218,553		220,406
Net income per share—basic																		$ 3.22		$ 2.83		$ 0.56
Net income per share—diluted		$ 0.79	[2]	$ 0.83	[2]	$ 0.78	[2]	$ 0.78	[2]	$ 0.64	[2]	$ 0.66	[2]	$ 0.96	[2]	$ 0.52	[2]	$ 3.20	[2]	$ 2.77	[2]	$ 0.55
Stock Compensation Plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Ant-dilutive shares - Shares of common stock equivalents																		221		337		353
Convertible Debt | Convertible Senior Notes Due 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Adjustment for dilutive Convertible Senior Notes due 2019, net of tax																		$ 0		$ 0		$ (215)
Convertible Debt | Convertible Senior Notes Due 2017 and 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Dilutive effect of Convertible Senior Notes due 2017 and 2019																		0		0		780

[1] The following number of shares of our common stock equivalents issued under our share-based compensation arrangements were not included in the calculation of diluted net income per share because they were anti-dilutive:   Year Ended December 31, (In thousands) 2019   2018   2017 Shares of common stock equivalents 221
  337
  353

[2] Diluted net income per share is computed independently for each period presented. Consequently, the sum of the quarters may not equal the total net income per share for the year.