Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional paid-in capital [Member]	Retained Earnings [Member]	Retained Earnings [Member] Accounting Standards Update 2016-02 [Member]	Retained Earnings [Member] Accounting Standards Update 2016-13 [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment	Accumulated other comprehensive income (loss) [Member]	Treasury stock [Member]
Beginning of year at Dec. 31, 2018		$ 205,697	$ 390,315	$ 1,858,414	$ 342	$ 0	$ 1,858,756	$ (77,956)	$ (584,668)
Dividend reinvestment plan		31	1,095
Stock purchase and compensation plans		1,115	20,937
Net income	$ 189,764			189,764
Dividends to stockholders				(36,021)
Other comprehensive income	176,382							176,382
Acquisition of treasury stock									(8,071)
End of period at Sep. 30, 2019	$ 2,137,376	206,843	412,347	2,012,499				98,426	(592,739)
Dividends declared per share to stockholders	$ 0.60
Common stock, shares outstanding, beginning of period at Dec. 31, 2018	58,948,554
Dividend reinvestment plan, shares	15,650
Stock purchase and compensation plan, shares	557,257
Acquisition of treasury stock, shares	(121,892)
Common stock, shares outstanding, end of period at Sep. 30, 2019	59,399,569
Beginning of year at Jun. 30, 2019		206,665	407,382	1,968,374	0	0	1,968,374	68,506	(591,383)
Dividend reinvestment plan		9	366
Stock purchase and compensation plans		169	4,599
Net income	$ 56,150			56,150
Dividends to stockholders				(12,025)
Other comprehensive income	29,920							29,920
Acquisition of treasury stock									(1,356)
End of period at Sep. 30, 2019	$ 2,137,376	206,843	412,347	2,012,499				98,426	(592,739)
Dividends declared per share to stockholders	$ 0.20
Common stock, shares outstanding, beginning of period at Jun. 30, 2019	59,328,108
Dividend reinvestment plan, shares	4,738
Stock purchase and compensation plan, shares	83,979
Acquisition of treasury stock, shares	(17,256)
Common stock, shares outstanding, end of period at Sep. 30, 2019	59,399,569
Beginning of year at Dec. 31, 2019	$ 2,194,936	206,968	418,521	2,080,529	0	1,435	2,081,964	81,750	(592,832)
Dividend reinvestment plan		44	1,216
Stock purchase and compensation plans		911	19,137
Net income	119,294			119,294
Dividends to stockholders				(41,661)
Other comprehensive income	105,340							105,340
Acquisition of treasury stock									(7,039)
End of period at Sep. 30, 2020	$ 2,393,613	207,923	438,874	2,159,597				187,090	(599,871)
Dividends declared per share to stockholders	$ 0.69
Common stock, shares outstanding, beginning of period at Dec. 31, 2019	59,461,153
Dividend reinvestment plan, shares	21,900
Stock purchase and compensation plan, shares	455,537
Acquisition of treasury stock, shares	(103,888)
Common stock, shares outstanding, end of period at Sep. 30, 2020	59,834,702
Beginning of year at Jun. 30, 2020		207,875	435,019	2,103,629	$ 0	$ 0	$ 2,103,629	151,966	(599,814)
Dividend reinvestment plan		14	401
Stock purchase and compensation plans		34	3,454
Net income	$ 69,875			69,875
Dividends to stockholders				(13,907)
Other comprehensive income	35,124							35,124
Acquisition of treasury stock									(57)
End of period at Sep. 30, 2020	$ 2,393,613	$ 207,923	$ 438,874	$ 2,159,597				$ 187,090	$ (599,871)
Dividends declared per share to stockholders	$ 0.23
Common stock, shares outstanding, beginning of period at Jun. 30, 2020	59,811,742
Dividend reinvestment plan, shares	7,026
Stock purchase and compensation plan, shares	16,936
Acquisition of treasury stock, shares	(1,002)
Common stock, shares outstanding, end of period at Sep. 30, 2020	59,834,702