Unaudited Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	Retained Earnings Substantially Restricted	Accumulated Other Comprehensive (Loss) Income
Balance, beginning, shares at Dec. 31, 2018		84,521,692
Balance, beginning at Dec. 31, 2018	$ 1,515,854	$ 845	$ 1,051,253	$ 473,183	$ (9,427)
Net income	101,524			101,524
Other comprehensive income	52,875				52,875
Cash dividends declared	(45,476)			(45,476)
Stock issued in connection with acquisitions, shares		2,046,341
Stock issued in connection with acquisitions, value	87,153	$ 20	87,133
Stock issuances under stock incentive plans, shares		69,361
Stock issuances under stock incentive plans, value	0	$ 1	(1)
Stock-based compensation and related taxes	904		904
Balance, ending, shares at Jun. 30, 2019		86,637,394
Balance, ending at Jun. 30, 2019	1,687,376	$ 866	1,139,289	503,773	43,448
Balance, beginning, shares at Mar. 31, 2019		84,588,199
Balance, beginning at Mar. 31, 2019	1,550,850	$ 846	1,051,299	474,818	23,887
Net income	52,392			52,392
Other comprehensive income	19,561				19,561
Cash dividends declared	(23,437)			(23,437)
Stock issued in connection with acquisitions, shares		2,046,341
Stock issued in connection with acquisitions, value	87,153	$ 20	87,133
Stock issuances under stock incentive plans, shares		2,854
Stock issuances under stock incentive plans, value	0	$ 0	0
Stock-based compensation and related taxes	857		857
Balance, ending, shares at Jun. 30, 2019		86,637,394
Balance, ending at Jun. 30, 2019	1,687,376	$ 866	1,139,289	503,773	43,448
Cumulative Effect of New Accounting Principle in Period of Adoption	$ (25,458)			(25,458)
Balance, beginning, shares at Dec. 31, 2019	92,289,750	92,289,750
Balance, beginning at Dec. 31, 2019	$ 1,960,733	$ 923	1,378,534	541,050	40,226
Net income	106,783			106,783
Other comprehensive income	89,683				89,683
Cash dividends declared	(55,451)			(55,451)
Stock issued in connection with acquisitions, shares		3,007,044
Stock issued in connection with acquisitions, value	112,133	$ 30	112,103
Stock issuances under stock incentive plans, shares		112,267
Stock issuances under stock incentive plans, value	0	$ 1	(1)
Stock-based compensation and related taxes	$ 2,181		2,181
Balance, ending, shares at Jun. 30, 2020	95,409,061	95,409,061
Balance, ending at Jun. 30, 2020	$ 2,203,715	$ 954	1,492,817	580,035	129,909
Balance, beginning, 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
Net income	63,444			63,444
Other comprehensive income	30,185				30,185
Cash dividends declared	(27,724)			(27,724)
Stock issuances under stock incentive plans, shares		787
Stock issuances under stock incentive plans, value	0	$ 0	0
Stock-based compensation and related taxes	$ 1,166		1,166
Balance, ending, shares at Jun. 30, 2020	95,409,061	95,409,061
Balance, ending at Jun. 30, 2020	$ 2,203,715	$ 954	$ 1,492,817	580,035	$ 129,909
Cumulative Effect of New Accounting Principle in Period of Adoption	$ (12,347)			$ (12,347)