Consolidated Statements of Changes in Shareholders' Equity (unaudited) - USD ($) $ in Thousands	Total	Common Stock	Capital Surplus	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principles	$ 6,322			$ 6,322
Adjusted balance	2,695,892	$ 175,141	$ 2,031,695	534,006	$ (44,950)
Beginning balance at Dec. 31, 2018	2,689,570	175,141	2,031,695	527,684	(44,950)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.13
Beginning balance at Dec. 31, 2018	$ 2,689,570	175,141	2,031,695	527,684	(44,950)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	119,240
Other comprehensive income (loss)	86,987
Ending balance at Jun. 30, 2019	$ 2,803,139	172,231	1,981,205	607,666	42,037
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.26
Beginning balance at Mar. 31, 2019	$ 2,751,872	173,979	2,007,962	567,311	2,620
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.13
Cumulative effect of change in accounting principles	$ (31,150)			(31,150)
Adjusted balance	2,821,303	169,616	1,944,445	651,035	56,207
Beginning balance at Dec. 31, 2019	2,852,453	169,616	1,944,445	682,185	56,207
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,640			22,640
Other comprehensive income (loss)	81,950				81,950
Dividends - common stock	(23,535)			(23,535)
Common stock issued	151	9	142
Common stock repurchased	(81,822)	(5,076)	(76,746)
Share-based compensation expense	2,748		2,748
Stock activity under incentive compensation plans	0	560	(329)	(231)
Ending balance at Mar. 31, 2020	$ 2,823,435	165,109	1,870,260	649,909	138,157
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.14
Beginning balance at Dec. 31, 2019	$ 2,852,453	169,616	1,944,445	682,185	56,207
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	74,345
Other comprehensive income (loss)	92,836
Ending balance at Jun. 30, 2020	$ 2,864,255	165,093	1,871,741	678,378	149,043
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.28
Beginning balance at Mar. 31, 2020	$ 2,823,435	165,109	1,870,260	649,909	138,157
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	51,705			51,705
Other comprehensive income (loss)	10,886				10,886
Dividends - common stock	(23,113)			(23,113)
Common stock issued	145	11	134
Common stock repurchased	(473)	(34)	(439)
Share-based compensation expense	1,480		1,480
Stock activity under incentive compensation plans	190	7	306	(123)
Ending balance at Jun. 30, 2020	$ 2,864,255	$ 165,093	$ 1,871,741	$ 678,378	$ 149,043
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per common share (in dollars per share)	$ 0.14