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	Columbia Bank MHC	Columbia Bank MHC Common Stock	Columbia Bank Foundation	Columbia Bank Foundation Common Stock	Columbia Bank Foundation Additional Paid-in-Capital	IPO	IPO Common Stock	IPO Additional Paid-in-Capital
Balance at beginning of period at Dec. 31, 2017	$ 472,070	$ 0	$ 0	$ 537,480	$ (65,410)	$ 0	$ 0	$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	7,869			7,869
Other comprehensive income (loss)	(16,360)				(16,360)
Issuance of common stock										$ 626	$ 626	$ 34,767	$ 35	$ 34,732	$ 491,802	$ 498	$ 491,304
Purchase of Employee Stock Ownership Plan shares	(45,428)						(45,428)
Employee Stock Ownership Plan shares committed to be released	1,700		680				1,020
Balance at end of year at Sep. 30, 2018	947,046	1,159	526,716	545,349	(81,770)	0	(44,408)	0	0
Balance at beginning of period at Jun. 30, 2018	941,297	1,159	526,332	534,522	(75,736)	0	(44,980)	0	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	10,827			10,827
Other comprehensive income (loss)	(6,034)				(6,034)
Employee Stock Ownership Plan shares committed to be released	956		384				572
Balance at end of year at Sep. 30, 2018	947,046	1,159	526,716	545,349	(81,770)	0	(44,408)	0	0
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
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
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	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