Accumulated Other Comprehensive Income Amounts reclassified from Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	[2]	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment securities gains (losses), net											$ (89)	$ (11)	$ 1,181
Income before income taxes	$ 55,482	$ 57,821	$ 51,748	$ 46,785	$ 40,851	[1]	$ 59,107		$ 38,252	$ 39,773	211,836	177,983	149,781
Income Tax Expense (Benefit)	(10,734)	(11,406)	(9,999)	(6,815)	(25,123)	[1]	(18,338)		(11,120)	(10,574)	(38,954)	(65,155)	(44,915)
Net income	$ 44,748	$ 46,415	$ 41,749	$ 39,970	15,728	[1]	$ 40,769		$ 27,132	$ 29,199	172,882	112,828	104,866
Compensation and employee benefits											200,199	169,674	150,282
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											106	(11)	1,181
Income before income taxes											106	(11)	1,181
Income Tax Expense (Benefit)											(25)	4	(429)
Net income											81	(7)	752
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											(319)	(350)	(668)
Income Tax Expense (Benefit)											74	127	243
Net income											(245)	(223)	(425)
Compensation and employee benefits											$ (319)	(350)	(668)
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
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)	$ (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 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 23, “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.