Condensed Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital In Excess Of Par Value [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2017		$ 8,238	$ 138,359	$ 2,130,257
Share repurchases		(12)		(17,287)
Cash dividends declared				(10,702)
Share-based compensation and restricted share issuances, net of taxes		3	562
Net income				109,333
Balance at Mar. 31, 2018	$ 2,358,751	8,229	138,921	2,211,601
Balance at Dec. 31, 2018	2,680,483	8,123	142,176	2,530,184
Share repurchases		(22)		(30,574)
Cash dividends declared				(13,792)
Share-based compensation and restricted share issuances, net of taxes		6	947
Net income				133,323
Balance at Mar. 31, 2019	$ 2,770,371	$ 8,107	$ 143,123	$ 2,619,141