Consolidated Statements of Cash Flows - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net income	$ 3,678,000	$ 22,736,000	$ 31,072,000	$ 32,953,000
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of deferred loan fees and costs, premiums and discounts	439,000	1,965,000	1,006,000	745,000
Net amortization of premiums and discounts on securities	328,000	1,212,000	1,460,000	2,013,000
Amortization Of Mortgage Servicing Rights After Additional Charges	22,000	(88,000)	105,000	105,000
Amortization of debt issuance costs	14,000	890,000	53,000	53,000
Depreciation and amortization of office properties and equipment	863,000	3,839,000	3,364,000	3,178,000
Provision for loan losses	3,400,000	6,677,000	6,426,000	417,000
(Gain) loss on securities available for sale	60,000	(116,000)	1,689,000	(355,000)
Proceeds from sales of loans held-for-sale	0	0	40,564,000	42,411,000
Origination of loans held-for-sale	0	0	(40,280,000)	(23,812,000)
(Gain) loss on sale of loans receivable, net	0	(618,000)	380,000	(655,000)
Loss (gain) on real estate owned, net	0	56,000	(233,000)	441,000
Loss on write-down of real estate owned	0	55,000	0	0
(Gain) loss on disposal of office properties and equipment	0	(5,000)	169,000	38,000
Deferred tax (benefit) expense	7,491,000	(5,490,000)	(1,426,000)	2,930,000
Increase in accrued interest receivable	(1,228,000)	(2,979,000)	(1,531,000)	(1,864,000)
Increase in other assets	(15,557,000)	(11,053,000)	(11,681,000)	(269,000)
Increase in accrued expenses and other liabilities	3,905,000	7,980,000	9,840,000	5,033,000
Income on bank-owned life insurance	(1,089,000)	(5,208,000)	(4,936,000)	(4,370,000)
Contribution of common stock to Columbia Bank Foundation	0	34,767,000	0	0
Employee stock ownership plan expense	0	2,594,000	0	0
Increase in deferred compensation obligations under Rabbi Trust	0	(620,000)	0	0
Net cash provided by operating activities	2,326,000	56,594,000	36,041,000	58,992,000
Cash flows from investing activities:
Proceeds from sales of securities available for sale	92,000	11,513,000	187,376,000	164,203,000
Proceeds from paydowns / maturities / calls of securities available for sale	7,009,000	69,977,000	68,409,000	96,956,000
Proceeds from paydowns / maturities / calls of securities held to maturity	1,845,000	8,820,000	769,000	0
Purchases of securities available for sale	(163,721,000)	(413,804,000)	(162,788,000)	(357,477,000)
Purchases of securities held to maturity	(108,640,000)	(31,639,000)	(30,484,000)	0
Proceeds from sales of loans held-for-sale	0	3,615,000	0	0
Proceeds from sales of loans receivable	0	32,039,000	62,407,000	28,624,000
Purchases of loans receivable	(56,095,000)	(32,251,000)	(20,473,000)	(21,149,000)
Net increase in loans receivable	(41,157,000)	(536,129,000)	(425,926,000)	(196,106,000)
Purchase of bank-owned life insurance	0	(30,000,000)	(4,500,000)	(6,000,000)
Death benefit proceeds from bank-owned life insurance	0	1,241,000	1,631,000	0
Proceeds from redemptions of Federal Home Loan Bank stock	6,476,000	67,035,000	33,193,000	16,560,000
Purchases of Federal Home Loan Bank stock	(15,296,000)	(81,309,000)	(35,035,000)	(16,138,000)
Proceeds from sales of office properties and equipment	0	8,000	17,000	0
Additions to office properties and equipment	(2,648,000)	(13,272,000)	(6,527,000)	(3,665,000)
Proceeds from sales of real estate owned	0	1,007,000	1,614,000	3,620,000
Net cash used in investing activities	(372,135,000)	(943,000)	(330,317,000)	(290,572,000)
Cash flows from financing activities:
Net increase in deposits	139,887,000	150,558,000	300,613,000	250,191,000
Proceeds from long-term borrowings	0	220,980,000	168,400,000	10,000,000
Payments on long-term borrowings	(90,000,000)	(210,000,000)	(90,000,000)	(55,000,000)
Net increase (decrease) in short-term borrowings	286,000,000	299,800,000	(27,400,000)	24,400,000
Payment for trust preferred securities	0	(51,547,000)	0	0
Increase (decrease) in advance payments by borrowers for taxes and insurance	(1,555,000)	6,467,000	(2,056,000)	4,505,000
Issuance of common stock	0	492,428,000	0	0
Purchase of employee stock ownership plan shares	0	(45,428,000)	0	0
Net cash provided by financing activities	334,332,000	863,258,000	349,557,000	234,096,000
Net (decrease) increase in cash and cash equivalents	(35,477,000)	(23,297,000)	55,281,000	2,516,000
Cash and cash equivalents at beginning of period	100,975,000	65,498,000	45,694,000	43,178,000
Cash and cash equivalents at end of period	65,498,000	42,201,000	100,975,000	45,694,000
Cash paid during the period for:
Interest on deposits and borrowings	11,484,000	61,987,000	44,397,000	44,545,000
Income tax payments, net	1,393,000	21,325,000	27,784,000	8,038,000
Non-cash investing and financing activities:
Transfer of loans receivable to real estate owned	566,000	251,000	515,000	2,278,000
Transfer of loans receivable to loans held-for-sale	0	11,696,000	0	0
Transfer of securities from available for sale to held to maturity	0	0	103,680,000	0
Securitization of loans	$ 0	$ 0	$ 0	$ 17,169,000