Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	COMMON STOCK	ADDITIONAL PAID-IN CAPITAL	RETAINED EARNINGS	RETAINED EARNINGS Cumulative Effect, Period of Adoption, Adjustment	ACCUMULATED OTHER COMPREHENSIVE INCOME	TREASURYSTOCK	UNALLOCATED ESOP SHARES	COMMON STOCK ACQUIRED BY DEFERRED COMP PLANS	DEFERRED COMPENSATION PLANS
Beginning Balance at Dec. 31, 2019	$ 1,413,840	$ (8,311)	$ 832	$ 1,007,303	$ 695,273	$ (8,311)	$ 3,821	$ (268,504)	$ (24,885)	$ (3,833)	$ 3,833
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	56,384				56,384
Other comprehensive income, net of tax	9,510						9,510
Cash dividends paid	(49,106)				(49,106)
Acquisition of supplemental director retirement plan	0									(1,336)	1,336
Distributions from DDFP	68			68						503	(503)
Purchases of treasury stock	(7,256)							(7,256)
Purchase of employee restricted shares to fund statutory tax withholding	(974)							(974)
Shares issued dividend reinvestment plan	451			50				401
Stock option exercises	0
Allocation of ESOP shares	2,127			(179)					2,306
Allocation of SAP shares	3,866			3,866
Treasury shares issued due to acquisition	180,828			(50,387)				231,215
Allocation of stock options	142			142
Ending Balance at Sep. 30, 2020	1,601,569		832	960,863	694,240		13,331	(45,118)	(22,579)	(4,666)	4,666
Beginning Balance at Jun. 30, 2020	1,410,407		832	1,009,978	685,509		12,794	(275,359)	(23,347)	(3,498)	3,498
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	27,143				27,143
Other comprehensive income, net of tax	537						537
Cash dividends paid	(18,412)				(18,412)
Acquisition of supplemental director retirement plan	0									(1,336)	1,336
Distributions from DDFP	16			16						168	(168)
Purchases of treasury stock	(961)							(961)
Purchase of employee restricted shares to fund statutory tax withholding	(13)							(13)
Allocation of ESOP shares	605			(163)					768
Allocation of SAP shares	1,371			1,371
Treasury shares issued due to acquisition	180,828			(50,387)				231,215
Allocation of stock options	48			48
Ending Balance at Sep. 30, 2020	1,601,569		832	960,863	694,240		13,331	(45,118)	(22,579)	(4,666)	4,666
Beginning Balance at Dec. 31, 2020	1,619,797		832	962,453	718,090		17,655	(59,018)	(20,215)	(4,549)	4,549
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	130,618				130,618
Other comprehensive income, net of tax	(9,898)						(9,898)
Cash dividends paid	(53,995)				(53,995)
Distributions from DDFP	107			107						425	(425)
Purchases of treasury stock	(13,913)							(13,913)
Purchase of employee restricted shares to fund statutory tax withholding	(961)							(961)
Stock option exercises	638			(82)				720
Allocation of ESOP shares	2,993			687					2,306
Allocation of SAP shares	3,887			3,887
Allocation of stock options	151			151
Ending Balance at Sep. 30, 2021	1,679,424		832	967,203	794,713		7,757	(73,172)	(17,909)	(4,124)	4,124
Beginning Balance at Jun. 30, 2021	1,677,634		832	965,470	775,235		14,082	(59,307)	(18,678)	(4,213)	4,213
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	37,268				37,268
Other comprehensive income, net of tax	(6,325)						(6,325)
Cash dividends paid	(17,790)				(17,790)
Distributions from DDFP	38			38						89	(89)
Purchases of treasury stock	(13,865)							(13,865)
Allocation of ESOP shares	994			225					769
Allocation of SAP shares	1,421			1,421
Allocation of stock options	49			49
Ending Balance at Sep. 30, 2021	$ 1,679,424		$ 832	$ 967,203	$ 794,713		$ 7,757	$ (73,172)	$ (17,909)	$ (4,124)	$ 4,124