Parent-only Financial Information - Statement of Cash Flows (Details) - USD ($)	3 Months Ended													12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2017
Cash flows from operating activities:
Net income	$ 13,549,000	$ 14,218,000	$ 12,031,000	$ 14,919,000	$ 14,867,000	$ 10,827,000	$ (14,742,000)	$ 11,784,000	$ 3,678,000	$ 1,514,000	$ 9,272,000	$ 10,293,000	$ 9,993,000	$ 54,717,000	$ 22,736,000		$ 31,072,000
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of debt issuance costs									14,000					0	890,000		53,000
Amortization of intangible assets									0					222,000	0		0
(Gain) loss on securities transactions									60,000					(2,612,000)	(116,000)		1,689,000
Change in fair value of equity securities									0					(305,000)	0		0
Deferred tax expense (benefit)									7,491,000					7,527,000	(5,490,000)		(1,426,000)
Increase in other assets									(15,557,000)					(51,856,000)	(11,053,000)		(11,681,000)
Increase in accrued expenses and other liabilities									3,905,000					3,032,000	7,980,000		9,840,000
Contribution of common stock to Columbia Bank Foundation									0					0	34,767,000		0
Net cash provided by operating activities									2,326,000					123,770,000	60,209,000		36,041,000
Cash flows from investing activities:
Proceeds from sales of debt securities available for sale									0					65,198,000	11,513,000	$ 0	187,376,000
Proceeds from sale of equity securities									92,000					1,065,000	0		0
Proceeds from paydowns / maturities / calls of debt securities available for sale									7,009,000					149,683,000	69,977,000		68,409,000
Purchases of debt securities available for sale									(163,721,000)					(176,171,000)	(413,804,000)		(162,788,000)
Purchase of equity securities									0					(416,000)	0		0
Loan to ESOP									0					0	(45,428,000)		0
Repayment of loan receivable from Bank									0					11,671,000	32,039,000		62,407,000
Net cash acquired in acquisition									0					(31,288,000)	0
Net cash used in investing activities									(372,135,000)					(622,960,000)	(946,764,000)		(330,317,000)
Cash flows from financing activities:
Purchase of treasury stock									0					(55,309,000)	0		0
Restricted stock forfeitures									0					(736,000)	0
Issuance of treasury stock allocated to restricted stock award grants									0					1,095,000	0
Net cash provided by financing activities									334,332,000					532,536,000	863,258,000		349,557,000
Net increase (decrease) in cash and cash equivalents									(35,477,000)					33,346,000	(23,297,000)		55,281,000
Cash and cash equivalents at beginning of period				42,201,000				65,498,000	100,975,000				45,694,000	42,201,000	65,498,000		45,694,000
Cash and cash equivalents at end of period	75,547,000				42,201,000				65,498,000	100,975,000				75,547,000	42,201,000	65,498,000	100,975,000
Non-cash assets acquired:
Debt securities available for sale									0					51,710,000	0		0
Equity securities									0					1,073,000	0		0
Other assets									0					767,000	0		0
Total non-cash assets acquired									0					856,640,000	0		0
Liabilities assumed:
Borrowings									0					82,761,000	0		0
Total liabilities assumed									0					880,399,000	0		0
Net non-cash liabilities acquired									0					(23,759,000)	0		0
Net cash acquired in acquisition									0					105,006,000	0		0
Parent Company
Cash flows from operating activities:
Net income									3,678,000					54,717,000	22,736,000		31,072,000
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of debt issuance costs									14,000					0	890,000		53,000
Amortization of intangible assets									0					(71,000)	0		0
(Gain) loss on securities transactions									60,000					(236,000)	0		0
Change in fair value of equity securities									0					(65,000)	0		0
Deferred tax expense (benefit)									42,000					1,453,000	(6,086,000)		1,000
Increase in other assets									(494,000)					(2,026,000)	1,515,000		1,404,000
Increase in accrued expenses and other liabilities									989,000					3,515,000	(1,498,000)		62,000
Contribution of common stock to Columbia Bank Foundation									0					0	34,767,000		0
Equity in undistributed earnings (loss) of subsidiary									(4,288,000)					123,142,000	(51,401,000)		(32,295,000)
Net cash provided by operating activities									1,000					180,429,000	923,000		297,000
Cash flows from investing activities:
Capital contribution to subsidiary									0					0	(246,420,000)		0
Proceeds from sales of debt securities available for sale									92,000					0	0		0
Proceeds from sale of equity securities									0					1,065,000	0		0
Proceeds from paydowns / maturities / calls of debt securities available for sale									10,000					500,000	1,601,000		0
Purchases of debt securities available for sale									0					0	(414,000)		0
Purchase of equity securities									0					(416,000)	0		0
Loan to ESOP									0					0	(45,428,000)		0
Repayment of loan receivable from Bank									0					1,457,000	989,000		0
Net cash acquired in acquisition									0					(135,410,000)	0		0
Net cash used in investing activities									102,000					(132,804,000)	(289,672,000)		0
Cash flows from financing activities:
Payments on trust preferred securities									0					0	(51,547,000)		0
Issuance of common stock in initial public offering									0					0	492,428,000		0
Purchase of treasury stock									0					(55,309,000)	0		0
Issuance of common stock allocated to restricted stock award grants									0					21,687,000	0		0
Restricted stock forfeitures									0					(736,000)	0		0
Issuance of treasury stock allocated to restricted stock award grants									0					1,269,000	0		0
Net cash provided by financing activities									0					(33,089,000)	440,881,000		0
Net increase (decrease) in cash and cash equivalents									103,000					14,536,000	152,132,000		297,000
Cash and cash equivalents at beginning of period				$ 153,833,000				$ 1,701,000	1,598,000				$ 1,301,000	153,833,000	1,701,000		1,301,000
Cash and cash equivalents at end of period	$ 168,369,000				$ 153,833,000				1,701,000	$ 1,598,000				168,369,000	153,833,000	$ 1,701,000	1,598,000
Non-cash assets acquired:
Debt securities available for sale									0					1,998,000	0		0
Equity securities									0					208,000	0		0
Other assets									0					1,492,000	0		0
Total non-cash assets acquired									0					3,698,000	0		0
Liabilities assumed:
Borrowings									0					23,901,000	0		0
Total liabilities assumed									0					23,901,000	0		0
Net non-cash liabilities acquired									0					(20,203,000)	0		0
Net cash acquired in acquisition									$ 0					$ 884,000	$ 0		$ 0