Unaudited Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Paid-in Capital	Retained Earnings Substantially Restricted	Retained Earnings Substantially Restricted Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Compre- hensive (Loss) Income
Balance, beginning (in shares) at Dec. 31, 2018			84,521,692
Balance, beginning at Dec. 31, 2018	$ 1,515,854	$ (25,458)	$ 845	$ 1,051,253	$ 473,183	$ (25,458)	$ (9,427)
Increase (Decrease) in Stockholders' Equity
Net income	153,134				153,134
Other comprehensive income	57,522						57,522
Cash dividends declared	(72,260)				(72,260)
Stock issued in connection with acquisitions (in shares)			7,519,617
Stock issued in connection with acquisitions	316,538		$ 75	316,463
Stock issuances under stock incentive plans (in shares)			139,309
Stock issuances under stock incentive plans	0		$ 2	(2)
Stock-based compensation and related taxes	8,071			8,071
Balance, ending (in shares) at Sep. 30, 2019			92,180,618
Balance, ending at Sep. 30, 2019	$ 1,953,401		$ 922	1,375,785	$ 528,599		48,095
Increase (Decrease) in Stockholders' Equity
Cash dividends declared per share (in dollars per share)	$ 0.82				$ 0.82
Balance, beginning (in shares) at Jun. 30, 2019			86,637,394
Balance, beginning at Jun. 30, 2019	$ 1,687,376		$ 866	1,139,289	$ 503,773		43,448
Increase (Decrease) in Stockholders' Equity
Net income	51,610				51,610
Other comprehensive income	4,647						4,647
Cash dividends declared	(26,784)				(26,784)
Stock issued in connection with acquisitions (in shares)			5,473,276
Stock issued in connection with acquisitions	229,385		$ 55	229,330
Stock issuances under stock incentive plans (in shares)			69,948
Stock issuances under stock incentive plans	0		$ 1	(1)
Stock-based compensation and related taxes	7,167			7,167
Balance, ending (in shares) at Sep. 30, 2019			92,180,618
Balance, ending at Sep. 30, 2019	$ 1,953,401		$ 922	1,375,785	$ 528,599		48,095
Increase (Decrease) in Stockholders' Equity
Cash dividends declared per share (in dollars per share)	$ 0.29				$ 0.29
Balance, beginning (in shares) at Dec. 31, 2019	92,289,750		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	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	0		$ 1	(1)
Stock-based compensation and related taxes	$ 3,292			3,292
Balance, ending (in shares) at Sep. 30, 2020	95,413,743		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				$ 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	1,189						1,189
Cash dividends declared	(28,683)				(28,683)
Stock issuances under stock incentive plans (in shares)			4,682
Stock issuances under stock incentive plans	0		$ 0	0
Stock-based compensation and related taxes	$ 1,111			1,111
Balance, ending (in shares) at Sep. 30, 2020	95,413,743		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				$ 0.30