Consolidated Statements of Cash Flows - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2017
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 deferred loan fees and costs, premiums and discounts	439,000	1,205,000	1,965,000	1,006,000
Net amortization of premiums and discounts on securities	328,000	1,160,000	1,212,000	1,460,000
Net amortization of mortgage servicing rights	22,000	(109,000)	(88,000)	105,000
Amortization of debt issuance costs	14,000	0	890,000	53,000
Amortization of intangible assets	0	222,000	0	0
Depreciation and amortization of office properties and equipment	863,000	4,880,000	3,839,000	3,364,000
Provision for loan losses	3,400,000	4,224,000	6,677,000	6,426,000
(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
Proceeds from sales of loans held-for-sale	0	101,946,000	3,615,000	40,564,000
Origination of loans held-for-sale	0	0	0	(40,280,000)
(Gain) loss on sale of loans	0	(785,000)	(618,000)	380,000
Loss (gain) on real estate owned	0	1,000	56,000	(233,000)
Loss on write-down of real estate owned	0	0	55,000	0
Loss (gain) on disposal of office properties and equipment	0	3,000	(5,000)	169,000
Deferred tax expense (benefit)	7,491,000	7,527,000	(5,490,000)	(1,426,000)
Increase in accrued interest receivable	(1,228,000)	(959,000)	(2,979,000)	(1,531,000)
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
Income on bank-owned life insurance	(1,089,000)	(5,846,000)	(5,208,000)	(4,936,000)
Contribution of common stock to Columbia Bank Foundation	0	0	34,767,000	0
Employee stock ownership plan expense	0	3,566,000	2,594,000	0
Stock based compensation	0	3,694,000	0	0
Increase in deferred compensation obligations under Rabbi Trust	0	65,000	(620,000)	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	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
Proceeds from paydowns / maturities / calls of debt securities held to maturity	1,845,000	46,133,000	8,820,000	769,000
Purchases of debt securities available for sale	(163,721,000)	(176,171,000)	(413,804,000)	(162,788,000)
Purchases of debt securities held to maturity	(108,640,000)	(70,126,000)	(31,639,000)	(30,484,000)
Purchase of equity securities	0	(416,000)	0	0
Proceeds from sales of loans receivable	0	11,671,000	32,039,000	62,407,000
Purchase of loans receivable	(56,095,000)	(89,774,000)	(32,251,000)	(20,473,000)
Net increase in loans receivable	(41,157,000)	(503,772,000)	(536,129,000)	(425,926,000)
Purchase of bank-owned life insurance	0	0	(30,000,000)	(4,500,000)
Proceeds from bank-owned life insurance death benefit	0	1,015,000	1,241,000	1,631,000
Proceeds from redemptions of Federal Home Loan Bank stock	6,476,000	72,871,000	67,035,000	33,193,000
Purchases of Federal Home Loan Bank stock	(15,296,000)	(79,796,000)	(81,309,000)	(35,035,000)
Proceeds from sales of office properties and equipment	0	0	8,000	17,000
Additions to office properties and equipment	(2,648,000)	(19,344,000)	(13,272,000)	(6,527,000)
Proceeds from sales of real estate owned	0	91,000	1,007,000	1,614,000
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:
Net increase in deposits	139,887,000	449,270,000	150,558,000	300,613,000
Proceeds from long-term borrowings	0	140,000,000	220,980,000	168,400,000
Payments on long-term borrowings	(90,000,000)	(230,000,000)	(210,000,000)	(90,000,000)
Net increase (decrease) in short-term borrowings	286,000,000	225,081,000	299,800,000	(27,400,000)
Payments on trust preferred securities	0	0	(51,547,000)	0
Increase (decrease) in advance payments by borrowers for taxes and insurance	(1,555,000)	3,121,000	6,467,000	(2,056,000)
Issuance of common stock	0	14,000	492,428,000	0
Purchase of treasury stock	0	(55,309,000)	0	0
Purchase of employee stock ownership plan shares	0	0	(45,428,000)	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	100,975,000	42,201,000	65,498,000	45,694,000
Cash and cash equivalents at end of period	65,498,000	75,547,000	42,201,000	100,975,000
Cash paid during the period for:
Interest on deposits and borrowings	11,484,000	87,370,000	61,987,000	44,397,000
Income tax payments, net of (refunds)	1,393,000	(2,893,000)	21,325,000	27,784,000
Non-cash investing and financing activities
Transfer of loans receivable to real estate owned	566,000	0	251,000	515,000
Transfer of loans receivable to loans held-for-sale	0	93,147,000	11,696,000	0
Transfer of securities from available for sale to held to maturity	0	0	0	103,680,000
Securitization of loans	0	21,615,000	0	0
Non-cash assets acquired:
Debt securities available for sale	0	51,710,000	0	0
Equity securities	0	1,073,000	0	0
Federal Home Loan Bank stock	0	3,716,000	0	0
Loans receivable	0	757,223,000	0	0
Accrued interest receivable	0	2,239,000	0	0
Office properties and equipment, net	0	6,815,000	0	0
Bank-owned life insurance	0	22,096,000	0	0
Deferred tax assets, net	0	3,534,000	0	0
Core deposit and other intangibles	0	7,467,000	0	0
Other assets	0	767,000	0	0
Total non-cash assets acquired	0	856,640,000	0	0
Liabilities assumed:
Deposits	0	782,698,000	0	0
Borrowings	0	82,761,000	0	0
Advance payments by borrowers for taxes and insurance	0	356,000	0	0
Accrued expenses and other liabilities	0	14,584,000	0	0
Total liabilities assumed	0	880,399,000	0	0
Net non-cash liabilities acquired	0	(23,759,000)	0	0
Net cash and cash equivalents acquired in acquisition	$ 0	$ 105,006,000	$ 0	$ 0