Parent-only Financial Information - Statement of Cash Flows (Details) - USD ($)	3 Months Ended													12 Months Ended
	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	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net income	$ 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	$ 8,497,000	$ 8,233,000	$ 8,313,000	$ 7,910,000	$ 22,736,000	$ 31,072,000	$ 32,953,000
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of debt issuance costs					14,000									890,000	53,000	53,000
Loss on securities available for sale					60,000									(116,000)	1,689,000	(355,000)
Deferred tax (benefit) expense					7,491,000									(5,490,000)	(1,426,000)	2,930,000
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
Contribution of common stock to Columbia Bank Foundation					0									34,767,000	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 calls and maturities					7,009,000									69,977,000	68,409,000	96,956,000
Loan to ESOP					0									(45,428,000)	0	0
Repayment of loans receivable from Bank					0									32,039,000	62,407,000	28,624,000
Net cash used in investing activities					(372,135,000)									(943,000)	(330,317,000)	(290,572,000)
Cash flows from financing activities:
Payment for trust preferred securities					0									(51,547,000)	0	0
Issuance of common stock					0									492,428,000	0	0
Net cash provided by financing activities					334,332,000									863,258,000	349,557,000	234,096,000
Net increase (decrease) 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				65,498,000	100,975,000				45,694,000				$ 43,178,000	65,498,000	45,694,000	43,178,000
Cash and cash equivalents at end of period	42,201,000				65,498,000	100,975,000				45,694,000				42,201,000	100,975,000	45,694,000
Parent Company
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 debt issuance costs					14,000									890,000	53,000	53,000
Loss on securities available for sale					60,000									0	0	0
Deferred tax (benefit) expense					42,000									(6,086,000)	1,000	(2,000)
Increase in other assets					(494,000)									1,515,000	1,404,000	(1,600,000)
Increase in accrued expenses and other liabilities					989,000									(1,498,000)	62,000	10,000
Contribution of common stock to Columbia Bank Foundation					0									34,767,000	0	0
Equity in undistributed earnings of subsidiary					(4,288,000)									(51,401,000)	(32,295,000)	(32,744,000)
Net cash provided by operating activities					1,000									923,000	297,000	(1,330,000)
Cash flows from investing activities:
Capital contribution to subsidiary					0									(246,420,000)	0	0
Proceeds from sales of securities available for sale					92,000									0	0	0
Proceeds from calls and maturities					10,000									1,601,000	0	0
Purchases of securities available for sale					0									(414,000)	0	0
Loan to ESOP					0									(45,428,000)	0	0
Repayment of loans receivable from Bank					0									989,000	0	0
Net cash used in investing activities					102,000									(289,672,000)	0	0
Cash flows from financing activities:
Payment for trust preferred securities					0									(51,547,000)
Issuance of common stock					0									492,428,000
Net cash provided by financing activities					0									440,881,000	0	0
Net increase (decrease) in cash and cash equivalents					103,000									152,132,000	297,000	(1,330,000)
Cash and cash equivalents at beginning of period				$ 1,701,000	1,598,000				$ 1,301,000					1,701,000	1,301,000
Cash and cash equivalents at end of period	$ 153,833,000				$ 1,701,000	$ 1,598,000				$ 1,301,000				$ 153,833,000	$ 1,598,000	$ 1,301,000