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 (loss)	(2,958)			(2,958)
Other comprehensive income (loss)	(10,326)				(10,326)
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	744		296				448
Balance at end of year at Jun. 30, 2018	941,297	1,159	526,332	534,522	(75,736)	0	(44,980)	0	0
Balance at beginning of period at Mar. 31, 2018	473,904	0	0	549,264	(75,360)	0	0	0	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(14,742)			(14,742)
Other comprehensive income (loss)	(376)				(376)
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	744		296				448
Balance at end of year at Jun. 30, 2018	941,297	1,159	526,332	534,522	(75,736)	0	(44,980)	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 (loss)	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 (loss)	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