Note 6 - Capital Stock (Details Textual) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended			9 Months Ended
	Oct. 22, 2020	Jul. 22, 2020	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jan. 22, 2020	Apr. 20, 2017
Stock Repurchase Program, Authorized Amount								$ 500,000	$ 500,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount			$ 520,000			$ 520,000
Treasury Stock, Shares, Acquired (in shares)				0		800,000
Treasury Stock, Value, Acquired, Cost Method			$ 0		$ 35,946	$ 75,193	$ 225,680
Common Stock, Dividends, Per Share, Declared (in dollars per share)		$ 0.27	$ 0.27		$ 0.26	$ 0.81	$ 0.78
Dividends Payable, Date Declared		Jul. 22, 2020
Dividends Payable, Date to be Paid		Aug. 21, 2020
Dividends Payable, Date of Record		Aug. 07, 2020
Subsequent Event [Member]
Common Stock, Dividends, Per Share, Declared (in dollars per share)	$ 0.27
Dividends Payable, Date Declared	Oct. 22, 2020