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, net of tax	Unallocated common stock held by the employee stock ownership plan	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 quarter at Dec. 31, 2016	$ 434,852	$ 0	$ 0	$ 501,014	$ (66,162)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	21,080	0	0	21,080	0
Other comprehensive income	19,982	0	0	0	19,982
Balance at end of year at Sep. 30, 2017	475,914	0	0	522,094	(46,180)	0
Balance at beginning of quarter at Jun. 30, 2017					(64,138)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,514
Other comprehensive income	17,958
Balance at end of year at Sep. 30, 2017	475,914	0	0	522,094	(46,180)	0
Balance at beginning of quarter at Dec. 31, 2017	472,070			537,480	(65,410)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	7,869			7,869	0
Other comprehensive income	(16,360)			0	(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)	(44,408)
Balance at beginning of quarter at Jun. 30, 2018					(75,736)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	10,827
Other comprehensive income	(6,034)
Balance at end of year at Sep. 30, 2018	$ 947,046	$ 1,159	$ 526,716	$ 545,349	$ (81,770)	$ (44,408)