Consolidated Statements of Changes in Stockholders' Equity - 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 Sep. 30, 2016	$ 439,664	$ 0	$ 0	$ 491,022	$ (51,358)		$ 0	$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	31,072			31,072
Other comprehensive income	5,178				5,178				0
Balance at end of year at Sep. 30, 2017	475,914	0	0	522,094	(46,180)		0	0	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	3,678			3,678
Other comprehensive income	(19,230)								0
Other comprehensive income (loss)	(7,522)				(7,522)
Balance at end of year at Dec. 31, 2017	472,070	0	0	537,480	(65,410)		0	0	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,736			22,736
Other comprehensive income	(6,487)				(6,487)
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	2,594		1,001				1,593
Funding of deferred compensation obligations	(620)							(1,259)	639
Balance at end of year 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	54,717			54,717
Other comprehensive income	3,710				3,710
Issuance of common stock allocated to restricted stock award grants	14	14	0
Treasury stock allocated to restricted stock award grants	0		(1,095)			1,095
Stock based compensation	3,694		3,694
Purchase of treasury stock (3,543,800 shares)	(55,309)					(55,309)
Restricted stock forfeitures	0		736			(736)
Employee Stock Ownership Plan shares committed to be released	3,566		1,295				2,271
Funding of deferred compensation obligations	65							(261)	326
Balance at end of year at Dec. 31, 2019	$ 982,517	$ 1,173	$ 531,667	$ 615,481	$ (68,735)	$ (54,950)	$ (41,564)	$ (1,520)	$ 965