Consolidated Statements Of Equity - USD ($) $ in Thousands	Common Stock [Member] Common Class A [Member]	Common Stock [Member]	Capital In Excess Of Par Value [Member] Tangible Equity Units [Member]	Capital In Excess Of Par Value [Member] Common Class A [Member]	Capital In Excess Of Par Value [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income [Member]	Tangible Equity Units [Member]	Total
Balance at Dec. 31, 2017		$ 90,350			$ 807,135	$ 1,132,556	$ (73,280)	$ 860		$ 1,957,621
Net income						50,839				50,839
Dividends declared						(36,386)				(36,386)
Issuance of common stock under dividend reinvestment plan		6			355					361
Repurchase of stock							(2,491)			(2,491)
Equity compensation plan		91			(91)
Exercise of stock options		31			979					1,010
Stock-based compensation					1,443	(41)				1,402
Cumulative effect of change in accounting principle - financial instruments						860		(860)
Other					(197)					(197)
Balance at Mar. 31, 2018		90,478			809,624	1,147,828	(75,771)			1,972,159
Balance at Dec. 31, 2017		90,350			807,135	1,132,556	(73,280)	$ 860		1,957,621
Net income										195,645
Balance at Sep. 30, 2018		90,499			814,004	1,217,007	(75,772)			2,045,738
Balance at Mar. 31, 2018		90,478			809,624	1,147,828	(75,771)			1,972,159
Net income						66,590				66,590
Dividends declared						(36,416)				(36,416)
Issuance of common stock under dividend reinvestment plan		5			354					359
Equity compensation plan		2			(2)
Exercise of stock options		1			8					9
Stock-based compensation					1,985	(128)				1,857
Other					(206)					(206)
Balance at Jun. 30, 2018		90,486			811,763	1,177,874	(75,771)			2,004,352
Net income						78,216				78,216
Dividends declared						(38,964)				(38,964)
Issuance of common stock under dividend reinvestment plan		5			376					381
Repurchase of stock							(1)			(1)
Equity compensation plan		3			(3)
Exercise of stock options		5			178					183
Stock-based compensation					1,899	(119)				1,780
Other					(209)					(209)
Balance at Sep. 30, 2018		90,499			814,004	1,217,007	(75,772)			2,045,738
Balance at Dec. 31, 2018		90,576			820,378	1,174,245	(75,835)			2,009,364
Net income						16,924				16,924
Dividends declared						(39,014)				(39,014)
Issuance of common stock under dividend reinvestment plan		59			3,976					4,035
Repurchase of stock							(1,857)			(1,857)
Equity compensation plan		67			(67)
Exercise of stock options		39			1,136					1,175
Stock-based compensation					1,929	42				1,971
Other					(13)					(13)
Balance at Mar. 31, 2019		90,741			827,339	1,152,197	(77,692)			1,992,585
Balance at Dec. 31, 2018		90,576			820,378	1,174,245	(75,835)			2,009,364
Net income										160,316
Balance at Sep. 30, 2019		109,471			2,633,193	1,197,600	(77,702)			3,862,562
Balance at Mar. 31, 2019		90,741			827,339	1,152,197	(77,692)			1,992,585
Net income						54,903				54,903
Dividends declared						(47,249)				(47,249)
Stock issued to finance pending acquisition		18,685			1,245,440					1,264,125
Proceeds from stock purchase contracts issued under tangible equity units			$ 557,837						$ 557,837
Issuance of common stock under dividend reinvestment plan		5			380					385
Repurchase of stock							(2)			(2)
Equity compensation plan		3			(3)
Exercise of stock options		10			361					371
Stock-based compensation					2,129	(98)				2,031
Other					(212)					(212)
Balance at Jun. 30, 2019		109,444			2,633,271	1,159,753	(77,694)			3,824,774
Net income						88,489				88,489
Dividends declared						(50,558)				(50,558)
Expenses incurred for equity offering and stock purchase contracts issued under tangible equity units					(1,474)					(1,474)
Issuance of common stock under dividend reinvestment plan		5			394					399
Issuance of common stock from stock purchase contracts	$ 15			$ (15)
Repurchase of stock							(8)			(8)
Equity compensation plan		2			(2)
Exercise of stock options		5			184					189
Stock-based compensation					1,774	(84)				1,690
Other					(939)					(939)
Balance at Sep. 30, 2019		$ 109,471			$ 2,633,193	$ 1,197,600	$ (77,702)			$ 3,862,562