Earnings Per Common Share (Schedule of Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017	[1]	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
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Basic earnings allocated to common shareholders																	$ 111,324		$ 103,310		$ 97,577
Net income	$ 15,728		$ 40,769		$ 27,132		$ 29,199		$ 30,718		$ 27,484		$ 25,405		$ 21,259		112,828		104,866		98,827
Earnings allocated to participating securities - preferred shares																	3		185		175
Earnings allocated to participating securities - nonvested restricted shares																	$ 1,501		$ 1,371		$ 1,075
Weighted average common shares outstanding																	59,882		57,184		57,019
Basic earnings per common share	$ 0.23	[3]	$ 0.70	[3]	$ 0.47	[3]	$ 0.50	[3]	$ 0.53	[3]	$ 0.47	[3]	$ 0.44	[3]	$ 0.37	[3]	$ 1.86	[3]	$ 1.81	[3]	$ 1.71
Earnings allocated to common shareholders, Diluted																	$ 111,324		$ 103,310		$ 97,577
Weighted average number of common shares outstanding																	59,882		57,184		57,019
Dilutive effect of equity awards and warrants																	6		9		13
Weighted average diluted common shares outstanding																	59,888		57,193		57,032
Diluted earnings per common share	$ 0.23	[3]	$ 0.70	[3]	$ 0.47	[3]	$ 0.50	[3]	$ 0.53	[3]	$ 0.47	[3]	$ 0.44	[3]	$ 0.37	[3]	$ 1.86	[3]	$ 1.81	[3]	$ 1.71
Potentially dilutive share options that were not included in the computation of diluted EPS because to do so would be anti-dilutive.																	13		19		37

[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] Due to averaging of shares, quarterly earnings per share may not add up to the totals reported for the full year.