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 (loss)
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	184,540				184,540
Other comprehensive income (loss)	90,872						90,872
Cash dividends declared	(84,134)				(84,134)
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)			116,949
Stock issuances under stock incentive plans			$ 1	(1)
Stock-based compensation and related taxes	3,292			3,292
Balance, ending (in shares) at Sep. 30, 2020			95,413,743
Balance, ending at Sep. 30, 2020	$ 2,255,089		$ 954	1,493,928	629,109		131,098
Increase (Decrease) in Stockholders' Equity
Cash dividends declared per share (in dollars per share)	$ 0.88
Balance, beginning (in shares) at Jun. 30, 2020			95,409,061
Balance, beginning at Jun. 30, 2020	$ 2,203,715		$ 954	1,492,817	580,035		129,909
Increase (Decrease) in Stockholders' Equity
Net income	77,757				77,757
Other comprehensive income (loss)	1,189						1,189
Cash dividends declared	(28,683)				(28,683)
Stock issuances under stock incentive plans (in shares)			4,682
Stock-based compensation and related taxes	1,111			1,111
Balance, ending (in shares) at Sep. 30, 2020			95,413,743
Balance, ending at Sep. 30, 2020	$ 2,255,089		$ 954	1,493,928	629,109		131,098
Increase (Decrease) in Stockholders' Equity
Cash dividends declared per share (in dollars per share)	$ 0.30
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	234,048				234,048
Other comprehensive income (loss)	(65,431)						(65,431)
Cash dividends declared	(90,929)				(90,929)
Stock issuances under stock incentive plans (in shares)			86,295
Stock issuances under stock incentive plans			$ 1	(1)
Stock-based compensation and related taxes	$ 2,887			2,887
Balance, ending (in shares) at Sep. 30, 2021	95,512,659		95,512,659
Balance, ending at Sep. 30, 2021	$ 2,387,616		$ 955	1,497,939	811,063		77,659
Increase (Decrease) in Stockholders' Equity
Cash dividends declared per share (in dollars per share)	$ 0.95
Balance, beginning (in shares) at Jun. 30, 2021			95,507,234
Balance, beginning at Jun. 30, 2021	$ 2,353,955		$ 955	1,496,488	766,070		90,442
Increase (Decrease) in Stockholders' Equity
Net income	75,619				75,619
Other comprehensive income (loss)	(12,783)						(12,783)
Cash dividends declared	(30,626)				(30,626)
Stock issuances under stock incentive plans (in shares)			5,425
Stock-based compensation and related taxes	$ 1,451			1,451
Balance, ending (in shares) at Sep. 30, 2021	95,512,659		95,512,659
Balance, ending at Sep. 30, 2021	$ 2,387,616		$ 955	$ 1,497,939	$ 811,063		$ 77,659
Increase (Decrease) in Stockholders' Equity
Cash dividends declared per share (in dollars per share)	$ 0.32