Consolidated Statements Of Equity - USD ($) $ in Thousands	Common Stock [Member]	Capital In Excess Of Par Value [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income [Member]	Total
Balance at Dec. 31, 2016	$ 90,155	$ 797,513	$ 1,032,844	$ (71,113)	$ 669	$ 1,850,068
Net income			239,738			239,738
Other comprehensive income, net of income tax of $102					191	191
Dividends declared			(140,660)			(140,660)
Issuance of common stock under dividend reinvestment plan	23	1,430				1,453
Repurchase of stock				(2,167)		(2,167)
Equity compensation plan	85	(85)
Exercise of stock options	87	2,786				2,873
Stock-based compensation		6,342	(348)			5,994
Other		(851)				(851)
Balance at Dec. 31, 2017	90,350	807,135	1,132,556	(73,280)	860	1,957,621
Cumulative effect of change in accounting principle - windfall tax benefit			982			982
Net income			191,988			191,988
Dividends declared			(150,736)			(150,736)
Issuance of common stock under dividend reinvestment plan	79	5,084				5,163
Repurchase of stock				(2,555)		(2,555)
Equity compensation plan	(101)	101
Exercise of stock options	46	1,413				1,459
Stock-based compensation		7,567	(423)			7,144
Other		(720)				(720)
Balance at Dec. 31, 2018	90,576	820,378	1,174,245	(75,835)		2,009,364
Cumulative effect of change in accounting principle - windfall tax benefit			860		$ (860)
Net income			224,543			224,543
Dividends declared			(188,512)			(188,512)
Stock issued to finance acquisition	18,685	1,244,414				1,263,099
Proceeds from stock purchase contracts issued under tangible equity units		557,389				557,389
Issuance of common stock from stock purchase contracts	2,423	(2,423)
Issuance of common stock under dividend reinvestment plan	118	8,841				8,959
Repurchase of stock				(1,867)		(1,867)
Equity compensation plan	73	(73)
Exercise of stock options	60	1,838				1,898
Stock-based compensation		7,368	(204)			7,164
Other		(1,177)				(1,177)
Balance at Dec. 31, 2019	$ 111,935	$ 2,636,555	$ 1,210,072	$ (77,702)		$ 3,880,860