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 (LOSS) INCOME	TREASURYSTOCK	UNALLOCATED ESOP SHARES	COMMON STOCK ACQUIRED BY DDFP	DEFERRED COMPENSATION DDFP
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of Adopting Accounting Standards Updates	$ 1,358,980	$ 4,350	$ 832	$ 1,021,533	$ 651,099	$ 4,350	$ (12,336)	$ (272,470)	$ (29,678)	$ (4,504)	$ 4,504
Beginning balance at Dec. 31, 2018	1,358,980	4,350	832	1,021,533	651,099	4,350	(12,336)	(272,470)	(29,678)	(4,504)	4,504
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	55,283				55,283
Other comprehensive income (loss), net of tax	16,312						16,312
Cash dividends paid	(44,317)				(44,317)
Effect of Adopting Accounting Standards Updates	1,358,980	4,350	832	1,021,533	651,099	4,350	3,976	(272,470)	(28,268)	(4,504)	4,504
Distributions from DDFP	85			85						335	(335)
Purchases of treasury stock	(4,050)							(4,050)
Purchase of employee restricted shares to fund statutory tax withholding	(1,953)							(1,953)
Shares issued dividend reinvestment plan	1,303			429				874
Stock option exercises	139			(96)				235
Allocation of ESOP shares	2,119			709					1,410
Allocation of SAP shares	3,107			3,107
Allocation of stock options	88			88
Ending Balance at Jun. 30, 2019	1,391,446		832	1,025,855	666,415		3,976	(277,364)	(28,268)	(4,169)	4,169
Beginning balance at Dec. 31, 2018	1,358,980	4,350	832	1,021,533	651,099	4,350	(12,336)	(272,470)	(29,678)	(4,504)	4,504
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of Adopting Accounting Standards Updates	1,413,840	(8,311)	832	1,007,303	695,273	(8,311)	3,821	(268,504)	(24,885)	(3,833)	3,833
Ending 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]
Accounting standards update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Effect of Adopting Accounting Standards Updates	$ 1,373,816		832	1,023,671	657,375		(5,084)	(274,005)	(28,973)	(4,337)	4,337
Beginning balance at Mar. 31, 2019	1,373,816		832	1,023,671	657,375		(5,084)	(274,005)	(28,973)	(4,337)	4,337
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	24,393				24,393
Other comprehensive income (loss), net of tax	9,060						9,060
Cash dividends paid	(15,353)				(15,353)
Effect of Adopting Accounting Standards Updates	1,391,446		832	1,025,855	666,415		3,976	(277,364)	(28,268)	(4,169)	4,169
Distributions from DDFP	43			43						168	(168)
Purchases of treasury stock	(3,870)							(3,870)
Purchase of employee restricted shares to fund statutory tax withholding	(39)							(39)
Shares issued dividend reinvestment plan	463			122				341
Stock option exercises	124			(85)				209
Allocation of ESOP shares	1,044			339					705
Allocation of SAP shares	1,719			1,719
Allocation of stock options	46			46
Ending Balance at Jun. 30, 2019	1,391,446		832	1,025,855	666,415		3,976	(277,364)	(28,268)	(4,169)	4,169
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of Adopting Accounting Standards Updates	1,391,446		832	1,025,855	666,415		3,976	(277,364)	(28,268)	(4,169)	4,169
Effect of Adopting Accounting Standards Updates	1,413,840	(8,311)	832	1,007,303	695,273	(8,311)	3,821	(268,504)	(24,885)	(3,833)	3,833
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	29,242				29,242
Other comprehensive income (loss), net of tax	8,973						8,973
Cash dividends paid	(30,695)				(30,695)
Effect of Adopting Accounting Standards Updates	1,413,840	$ (8,311)	832	1,009,978	685,509	$ (8,311)	12,794	(275,359)	(24,885)	(3,498)	3,498
Distributions from DDFP	52			52						335	(335)
Purchases of treasury stock	(6,295)							(6,295)
Purchase of employee restricted shares to fund statutory tax withholding	(961)							(961)
Shares issued dividend reinvestment plan	451			50				401
Allocation of ESOP shares	1,522			(16)					1,538
Allocation of SAP shares	2,495			2,495
Allocation of stock options	94			94
Ending 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]
Effect of Adopting Accounting Standards Updates	1,412,589		832	1,008,582	686,397		14,938	(274,044)	(24,116)	(3,666)	3,666
Beginning balance at Mar. 31, 2020	1,412,589		832	1,008,582	686,397		14,938	(274,044)	(24,116)	(3,666)	3,666
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	14,311				14,311
Other comprehensive income (loss), net of tax	(2,144)						(2,144)
Cash dividends paid	(15,199)				(15,199)
Effect of Adopting Accounting Standards Updates	1,412,589		832	1,009,978	685,509		12,794	(275,359)	(23,347)	(3,498)	3,498
Distributions from DDFP	15			15						168	(168)
Purchases of treasury stock	(1,310)							(1,310)
Purchase of employee restricted shares to fund statutory tax withholding	(5)							(5)
Allocation of ESOP shares	601			(168)					769
Allocation of SAP shares	1,502			1,502
Allocation of stock options	47			47
Ending 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]
Effect of Adopting Accounting Standards Updates	$ 1,410,407		$ 832	$ 1,009,978	$ 685,509		$ 12,794	$ (275,359)	$ (23,347)	$ (3,498)	$ 3,498