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)	$ 0	$ 0	$ (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	0	0	0	10,751	(10,751)
Net income	47,983	0	0	47,983	0
Other comprehensive income (loss)	(451)	0	0	0	(451)
Dividends - common stock	(19,782)	0	0	(19,782)	0
Common stock issued	105	6	99	0	0
Common stock repurchased	(1,115)	(64)	(1,051)	0	0
Share-based compensation expense	1,931	0	1,931	0	0
Stock activity under incentive compensation plans	229	190	298	(259)	0
Ending Balance at Mar. 31, 2018	2,179,118	152,172	1,640,776	447,696	(61,526)
Cumulative effect of change in accounting principles					(52)
Cumulative effect of change in accounting principles	6,322	0	0	6,322	0
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	0	0	56,276	0
Other comprehensive income (loss)	47,570	0	0	0	47,570
Dividends - common stock	(22,812)	0	0	(22,812)	0
Common stock issued	130	9	121	0	0
Common stock repurchased	(27,297)	(1,655)	(25,642)	0	0
Share-based compensation expense	1,800	0	1,800	0	0
Stock activity under incentive compensation plans	313	484	(12)	(159)	0
Ending Balance at Mar. 31, 2019	$ 2,751,872	$ 173,979	$ 2,007,962	$ 567,311	$ 2,620