Accumulated Other Comprehensive Income Amounts reclassified from Accumulated Other Comprehensive Income (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
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment securities gains (losses), net											$ (11)	$ 1,181	$ 1,581
Income before income taxes	$ 40,851	[1]	$ 59,107		$ 38,252	$ 39,773	$ 43,035	$ 39,608	$ 36,650	$ 30,488	177,983	149,781	141,620
Income Tax Expense (Benefit)	(25,123)	[1]	(18,338)		(11,120)	(10,574)	(12,317)	(12,124)	(11,245)	(9,229)	(65,155)	(44,915)	(42,793)
Net income	15,728	[1]	$ 40,769		$ 27,132	$ 29,199	$ 30,718	$ 27,484	$ 25,405	$ 21,259	112,828	104,866	98,827
Compensation and employee benefits											169,674	150,282	149,410
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment securities gains (losses), net											(11)	1,181	1,581
Income before income taxes											(11)	1,181	1,581
Income Tax Expense (Benefit)											4	(429)	(574)
Net income											(7)	752	1,007
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes											(350)	(668)	(336)
Income Tax Expense (Benefit)											127	243	122
Net income											(223)	(425)	(214)
Compensation and employee benefits											(350)	$ (668)	$ (336)
Accounting Standards Update 2018-02 [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	0										0
Accounting Standards Update 2018-02 [Member] | Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(4,082)										(4,082)
Accounting Standards Update 2018-02 [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(3,691)										(3,691)
Accounting Standards Update 2018-02 [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ (391)										$ (391)

[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.