Unaudited Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative-effect of accounting changes	Common Stock	Paid-in Capital	Retained Earnings Substantially Restricted	Retained Earnings Substantially Restricted Cumulative-effect of accounting changes	Accumulated Other Compre- hensive Income
Balance, beginning (in shares) at Dec. 31, 2019			92,289,750
Balance, beginning at Dec. 31, 2019	$ 1,960,733	$ (12,347)	$ 923	$ 1,378,534	$ 541,050	$ (12,347)	$ 40,226
Increase (Decrease) in Stockholders' Equity
Net income	106,783				106,783
Other comprehensive income (loss)	89,683						89,683
Cash dividends declared	(55,451)				(55,451)
Stock issued in connection with acquisitions (in shares)			3,007,044
Stock issued in connection with acquisitions	112,133		$ 30	112,103
Stock issuances under stock incentive plans (in shares)			112,267
Stock issuances under stock incentive plans			$ 1	(1)
Stock-based compensation and related taxes	2,181			2,181
Balance, ending (in shares) at Jun. 30, 2020			95,409,061
Balance, ending at Jun. 30, 2020	2,203,715		$ 954	1,492,817	580,035		129,909
Balance, beginning (in shares) at Mar. 31, 2020			95,408,274
Balance, beginning at Mar. 31, 2020	2,136,644		$ 954	1,491,651	544,315		99,724
Increase (Decrease) in Stockholders' Equity
Net income	63,444				63,444
Other comprehensive income (loss)	30,185						30,185
Cash dividends declared	(27,724)				(27,724)
Stock issuances under stock incentive plans (in shares)			787
Stock-based compensation and related taxes	1,166			1,166
Balance, ending (in shares) at Jun. 30, 2020			95,409,061
Balance, ending at Jun. 30, 2020	$ 2,203,715		$ 954	1,492,817	580,035		129,909
Balance, beginning (in shares) at Dec. 31, 2020	95,426,364		95,426,364
Balance, beginning at Dec. 31, 2020	$ 2,307,041		$ 954	1,495,053	667,944		143,090
Increase (Decrease) in Stockholders' Equity
Net income	158,429				158,429
Other comprehensive income (loss)	(52,648)						(52,648)
Cash dividends declared	(60,303)				(60,303)
Stock issuances under stock incentive plans (in shares)			80,870
Stock issuances under stock incentive plans			$ 1	(1)
Stock-based compensation and related taxes	$ 1,436			1,436
Balance, ending (in shares) at Jun. 30, 2021	95,507,234		95,507,234
Balance, ending at Jun. 30, 2021	$ 2,353,955		$ 955	1,496,488	766,070		90,442
Balance, beginning (in shares) at Mar. 31, 2021			95,501,819
Balance, beginning at Mar. 31, 2021	2,295,385		$ 955	1,495,438	719,072		79,920
Increase (Decrease) in Stockholders' Equity
Net income	77,627				77,627
Other comprehensive income (loss)	10,522						10,522
Cash dividends declared	(30,629)				(30,629)
Stock issuances under stock incentive plans (in shares)			5,415
Stock-based compensation and related taxes	$ 1,050			1,050
Balance, ending (in shares) at Jun. 30, 2021	95,507,234		95,507,234
Balance, ending at Jun. 30, 2021	$ 2,353,955		$ 955	$ 1,496,488	$ 766,070		$ 90,442