Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common stock	Additional paid-in-capital	Retained earnings	Accumulated other comprehensive loss, net of tax	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, 2015	$ 417,998	$ 0	$ 0	$ 458,069	$ (40,071)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	32,953			32,953
Other comprehensive loss	(11,287)				(11,287)
Balance at end of year 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 loss	5,178				5,178
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 loss	(19,230)
Other comprehensive 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]
Reclassification of tax effects resulting from the adoption of ASU No. 2018-02	0			11,708	(11,708)
Net income	22,736			22,736
Other comprehensive loss	(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)	$ (43,835)	$ (1,259)	$ 639
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of tax effects resulting from the adoption of ASU No. 2018-02					$ 0