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 year at Dec. 31, 2016	$ 434,852	$ 0	$ 0	$ 501,014	$ (66,162)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	19,567	0	0	19,567	0
Other comprehensive income	2,024	0	0	0	2,024
Balance at end of year at Jun. 30, 2017	456,443	0	0	520,581	(64,138)	0
Balance at beginning of year at Mar. 31, 2017					(65,910)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	9,272
Other comprehensive income	1,772
Balance at end of year at Jun. 30, 2017	456,443	0	0	520,581	(64,138)	0
Balance at beginning of year at Dec. 31, 2017	472,070			537,480	(65,410)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(2,958)			(2,958)	0
Other comprehensive income	(10,326)			0	(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)	(44,980)
Balance at beginning of year at Mar. 31, 2018					(75,360)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(14,742)
Other comprehensive income	(376)
Balance at end of year at Jun. 30, 2018	$ 941,297	$ 1,159	$ 526,332	$ 534,522	$ (75,736)	$ (44,980)