FDIC Loss-sharing Asset and Covered Assets (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended										12 Months Ended
		Dec. 31, 2017		Sep. 30, 2017	[2]	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Covered Assets And FDIC Loss Sharing Asset [Line Items]
Payment from (to) FDIC to terminate loss-sharing agreements												$ 4,666	$ 0	$ 0
Provision for loan and lease losses		$ 3,327	[1]	$ (648)		$ 3,177	$ 2,775	$ 18	$ 1,866	$ 3,640	$ 5,254	8,631	10,778	8,591
FDIC Indemnification Asset		$ 0						$ 3,535				0	3,535	6,568
FDIC Loss-sharing Asset, Cash Payments Paid to (Received from) the FDIC, net												(184)	(705)
FDIC Loss-sharing Asset, Net Reimbursable Losses												(149)	(1,153)
FDIC Loss-sharing Asset, Amortization, Net	[3]											(414)	(2,829)
FDIC Loss-sharing Asset, Impairment (Recapture) of Loans	[3]											40	301
FDIC Loss-sharing Asset, Disposals	[3]											(18)	(148)
FDIC Loss-sharing Asset, Write-downs (Write-ups) of Other Real Estate	[3]											0	(22)
FDIC Loss-sharing Asset, Other	[3]											(91)	(183)
Non-Single Family Covered Assets							74,000
Single Family Covered Assets							$ 26,400
FDIC indemnification asset settlement						(3,100)						(3,123)	0
FDIC clawback liability removal upon termination of loss-sharing agreements						$ 5,400
FDIC loss-sharing agreement early termination, pretax charge												$ 2,409	$ 0	$ 0

[1] During the fourth quarter of 2017, Columbia acquired Pacific Continental Corporation and also recorded a charge through provision for income taxes related to the re-measurement of our deferred tax assets pursuant to the newly enacted Tax Cuts and Jobs Act. See Note 2, “Business Combinations” for further information regarding this acquisition. See Note 24, “Income Tax” for further information regarding the re-measurement of our deferred tax assets.

[2] During the third quarter of 2017, Columbia sold its merchant card services portfolio. See Note 1, “Summary of Significant Accounting Policies” for further information regarding this transaction.

[3] Amounts shown in the table above for adjustments reflected in noninterest income include only those adjustments recorded to the noninterest income line item “Change in FDIC loss-sharing asset” in the Consolidated Statements of Income and do not include the charge related to the termination of the FDIC loss-sharing agreements.