Consolidated Statements of Changes in Stockholder's Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in-Capital	Retained Earnings	Accumulated Other Comprehensive (Loss)	Treasury Stock	Common Stock Held by the Employee Stock Ownership Plan	Stock Held by Rabbi Trust	Deferred Compensation Obligations	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjustment Retained Earnings	Cumulative Effect, Period of Adoption, Adjustment Accumulated Other Comprehensive (Loss)	Cumulative Effect, Period of Adoption, Adjusted Balance	Cumulative Effect, Period of Adoption, Adjusted Balance Common Stock	Cumulative Effect, Period of Adoption, Adjusted Balance Additional Paid-in-Capital	Cumulative Effect, Period of Adoption, Adjusted Balance Retained Earnings	Cumulative Effect, Period of Adoption, Adjusted Balance Accumulated Other Comprehensive (Loss)	Cumulative Effect, Period of Adoption, Adjusted Balance Treasury Stock	Cumulative Effect, Period of Adoption, Adjusted Balance Common Stock Held by the Employee Stock Ownership Plan	Cumulative Effect, Period of Adoption, Adjusted Balance Stock Held by Rabbi Trust	Cumulative Effect, Period of Adoption, Adjusted Balance Deferred Compensation Obligations
Balance at beginning of period at Dec. 31, 2018	$ 972,060	$ 1,159	$ 527,037	$ 560,216	$ (71,897)	$ 0	$ (43,835)	$ (1,259)	$ 639	$ 0	$ 548	$ (548)	$ 972,060	$ 1,159	$ 527,037	$ 560,764	$ (72,445)	$ 0	$ (43,835)	$ (1,259)	$ 639
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	41,168			41,168
Other comprehensive income (loss)	16,473				16,473
Issuance of common stock allocated to restricted stock award grants	14	14
Stock based compensation	1,630		1,630
Purchase of treasury stock	(42,084)					(42,084)
Employee Stock Ownership Plan shares committed to be released	2,619		921				1,698
Funding of deferred compensation obligations	(45)							(208)	163
Balance at end of year at Sep. 30, 2019	$ 991,835	1,173	529,588	601,932	(55,972)	(42,084)	(42,137)	(1,467)	802
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased, number of shares (in shares)	2,742,300
Balance at beginning of period at Jun. 30, 2019	$ 1,007,909	1,159	527,650	587,714	(61,319)	(3,867)	(42,710)	(1,407)	689
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	14,218			14,218
Other comprehensive income (loss)	5,347				5,347
Issuance of common stock allocated to restricted stock award grants	14	14
Stock based compensation	1,630		1,630
Purchase of treasury stock	(38,217)					(38,217)
Employee Stock Ownership Plan shares committed to be released	881		308				573
Funding of deferred compensation obligations	53							(60)	113
Balance at end of year at Sep. 30, 2019	$ 991,835	1,173	529,588	601,932	(55,972)	(42,084)	(42,137)	(1,467)	802
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased, number of shares (in shares)	2,478,400
Balance at beginning of period at Dec. 31, 2019	$ 982,517	1,173	531,667	615,481	(68,735)	(54,950)	(41,564)	(1,520)	965								$ (68,735)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	36,949			36,949
Other comprehensive income (loss)	(19,194)				(19,194)
Issuance of common stock to Columbia Bank MHC	68,530	47	68,483
Stock based compensation	6,585		6,585
Purchase of treasury stock	(60,286)					(60,286)
Restricted stock forfeitures	(16)		173			(189)
Repurchase shares for taxes	0		155			(155)
Employee Stock Ownership Plan shares committed to be released	2,374		672				1,702
Funding of deferred compensation obligations	(129)							(294)	165
Balance at end of year at Sep. 30, 2020	$ 1,017,330	1,220	607,735	652,430	(87,929)	(115,580)	(39,862)	(1,814)	1,130
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Shares repurchased, number of shares (in shares)	4,081,132
Restricted stock, shares forfeited (in shares)	16,511
Repurchased shares for taxes (in shares)	11,782
Balance at beginning of period at Jun. 30, 2020	$ 1,040,988	1,220	605,124	637,343	(53,207)	(108,327)	(40,434)	(1,767)	1,036
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	15,087			15,087
Other comprehensive income (loss)	(34,722)				(34,722)
Stock based compensation	2,176		2,176
Purchase of treasury stock	(6,925)					(6,925)
Restricted stock forfeitures	0		173			(173)
Repurchase shares for taxes	0		155			(155)
Employee Stock Ownership Plan shares committed to be released	679		107				572
Funding of deferred compensation obligations	47							(47)	94
Balance at end of year at Sep. 30, 2020	$ 1,017,330	$ 1,220	$ 607,735	$ 652,430	$ (87,929)	$ (115,580)	$ (39,862)	$ (1,814)	$ 1,130
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased, number of shares (in shares)	624,932
Restricted stock, shares forfeited (in shares)	15,390
Repurchased shares for taxes (in shares)	11,782