Note 6 - Capital Stock (Details Textual) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands			3 Months Ended		6 Months Ended
	Jul. 18, 2019	Apr. 18, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Apr. 20, 2017
Stock Repurchase Program, Authorized Amount							$ 500,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount			$ 181,000		$ 181,000
Treasury Stock, Shares, Acquired					1,980
Treasury Stock, Value, Acquired, Cost Method			$ 189,734	$ 50,027	$ 189,734	$ 50,027
Common Stock, Dividends, Per Share, Declared		$ 0.26	$ 0.26	$ 0.24	$ 0.52	$ 0.48
Dividends Payable, Date to be Paid		May 17, 2019
Dividends Payable, Date of Record		May 03, 2019
Subsequent Event [Member]
Common Stock, Dividends, Per Share, Declared	$ 0.26
Dividends Payable, Date to be Paid	Aug. 16, 2019
Dividends Payable, Date of Record	Aug. 02, 2019