Consolidated Statements of Changes in Shareholders' Equity (unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Cumulative effect of change in accounting principles	$ (4,179)			$ (4,127)	$ (52)
Beginning Balance at Dec. 31, 2017	2,154,397	$ 152,040	$ 1,639,499	413,130	(50,272)
Balance at Dec. 31, 2017	2,150,218	152,040	1,639,499	409,003	(50,324)
Reclassification of certain tax effects related to the Tax Cuts and Jobs Act of 2017				10,751	(10,751)
Net income	47,983			47,983
Other comprehensive income (loss)	(451)				(451)
Dividends - common stock	(19,782)			(19,782)
Common stock issued	105	6	99
Common stock repurchased	(1,115)	(64)	(1,051)
Share-based compensation expense	1,931		1,931
Stock activity under incentive compensation plans	229	190	298	(259)
Ending Balance at Mar. 31, 2018	2,179,118	152,172	1,640,776	447,696	(61,526)
Beginning Balance at Dec. 31, 2017	2,154,397	152,040	1,639,499	413,130	(50,272)
Balance at Dec. 31, 2017	2,150,218	152,040	1,639,499	409,003	(50,324)
Reclassification of certain tax effects related to the Tax Cuts and Jobs Act of 2017					(10,751)
Net income	143,332
Other comprehensive income (loss)	(18,855)
Ending Balance at Sep. 30, 2018	2,220,680	152,352	1,645,111	503,147	(79,930)
Beginning Balance at Mar. 31, 2018	2,179,118	152,172	1,640,776	447,696	(61,526)
Net income	44,001			44,001
Other comprehensive income (loss)	(5,177)				(5,177)
Dividends - common stock	(19,806)			(19,806)
Common stock issued	117	8	109
Common stock repurchased	(634)	(36)	(598)
Share-based compensation expense	1,675		1,675
Stock activity under incentive compensation plans	921	207	828	(114)
Ending Balance at Jun. 30, 2018	2,200,215	152,351	1,642,790	471,777	(66,703)
Net income	51,348			51,348
Other comprehensive income (loss)	(13,227)				(13,227)
Dividends - common stock	(19,806)			(19,806)
Common stock issued	149	8	141
Common stock repurchased	(27)	(2)	(25)
Share-based compensation expense	2,023		2,023
Stock activity under incentive compensation plans	5	(5)	182	(172)
Ending Balance at Sep. 30, 2018	2,220,680	152,352	1,645,111	503,147	(79,930)
Cumulative effect of change in accounting principles					(52)
Cumulative effect of change in accounting principles	6,322			6,322
Beginning Balance at Dec. 31, 2018	2,689,570	175,141	2,031,695	527,684	(44,950)
Balance at Dec. 31, 2018	2,695,892	175,141	2,031,695	534,006	(44,950)
Net income	56,276			56,276
Other comprehensive income (loss)	47,570				47,570
Dividends - common stock	(22,812)			(22,812)
Common stock issued	130	9	121
Common stock repurchased	(27,297)	(1,655)	(25,642)
Share-based compensation expense	1,800		1,800
Stock activity under incentive compensation plans	313	484	(12)	(159)
Ending Balance at Mar. 31, 2019	2,751,872	173,979	2,007,962	567,311	2,620
Beginning Balance at Dec. 31, 2018	2,689,570	175,141	2,031,695	527,684	(44,950)
Balance at Dec. 31, 2018	2,695,892	175,141	2,031,695	534,006	(44,950)
Net income	189,021
Other comprehensive income (loss)	103,464
Ending Balance at Sep. 30, 2019	2,832,530	170,031	1,948,808	655,177	58,514
Beginning Balance at Mar. 31, 2019	2,751,872	173,979	2,007,962	567,311	2,620
Net income	62,964			62,964
Other comprehensive income (loss)	39,417				39,417
Dividends - common stock	(22,476)			(22,476)
Common stock issued	148	10	138
Common stock repurchased	(30,845)	(1,884)	(28,961)
Share-based compensation expense	1,946		1,946
Stock activity under incentive compensation plans	113	126	120	(133)
Ending Balance at Jun. 30, 2019	2,803,139	172,231	1,981,205	607,666	42,037
Net income	69,781			69,781
Other comprehensive income (loss)	16,477				16,477
Dividends - common stock	(22,136)			(22,136)
Common stock issued	141	8	133
Common stock repurchased	(37,072)	(2,206)	(34,866)
Share-based compensation expense	2,200		2,200
Stock activity under incentive compensation plans		(2)	136	(134)
Ending Balance at Sep. 30, 2019	$ 2,832,530	$ 170,031	$ 1,948,808	$ 655,177	$ 58,514