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 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	$ 527,037	$ 560,764	$ (72,445)	$ 0	$ (43,835)	$ (1,259)	$ 639
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	26,950			26,950
Other comprehensive income (loss)	11,126				11,126
Purchase of treasury stock	(3,867)					(3,867)
Employee Stock Ownership Plan shares committed to be released	1,738		613				1,125
Funding of deferred compensation obligations	(98)							(148)	50
Balance at end of year at Jun. 30, 2019	1,007,909	1,159	527,650	587,714	(61,319)	(3,867)	(42,710)	(1,407)	689
Balance at beginning of period at Mar. 31, 2019	994,502	1,159	527,346	575,683	(65,820)	0	(43,276)	(1,359)	769
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	12,031			12,031
Other comprehensive income (loss)	4,501				4,501
Purchase of treasury stock	(3,867)					(3,867)
Employee Stock Ownership Plan shares committed to be released	870		304				566
Funding of deferred compensation obligations	(128)							(48)	(80)
Balance at end of year at Jun. 30, 2019	1,007,909	1,159	527,650	587,714	(61,319)	(3,867)	(42,710)	(1,407)	689
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	21,862			21,862
Other comprehensive income (loss)	15,528				15,528
Issuance of common stock to Columbia Bank MHC	68,530	47	68,483
Stock based compensation	4,409		4,409
Purchase of treasury stock	(53,361)					(53,361)
Restricted stock forfeitures	(16)					(16)
Employee Stock Ownership Plan shares committed to be released	1,695		565				1,130
Funding of deferred compensation obligations	(176)							(247)	71
Balance at end of year at Jun. 30, 2020	1,040,988	1,220	605,124	637,343	(53,207)	(108,327)	(40,434)	(1,767)	1,036
Balance at beginning of period at Mar. 31, 2020	961,126	1,173	534,213	622,246	(59,665)	(95,326)	(40,999)	(1,728)	1,212
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	15,097			15,097
Other comprehensive income (loss)	6,458				6,458
Issuance of common stock to Columbia Bank MHC	68,530	47	68,483
Stock based compensation	2,205		2,205
Purchase of treasury stock	(12,998)					(12,998)
Restricted stock forfeitures	(3)					(3)
Employee Stock Ownership Plan shares committed to be released	788		223				565
Funding of deferred compensation obligations	(215)							(39)	(176)
Balance at end of year at Jun. 30, 2020	$ 1,040,988	$ 1,220	$ 605,124	$ 637,343	$ (53,207)	$ (108,327)	$ (40,434)	$ (1,767)	$ 1,036