Parent Company Financial Information (Condensed Statements of Income) (Details) - USD ($) $ 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
Income
Deposits in banks											$ 813	$ 216	$ 109
Expense
Compensation and employee benefits											169,674	150,282	149,410
Interest Expense, Subordinated Notes and Debentures											304	0	0
Other borrowings											575	545	553
Other expense											34,161	28,362	30,521
Income before income tax benefit and equity in undistributed (excess distributed) earnings of subsidiaries	$ 40,851		$ 59,107		$ 38,252	$ 39,773	$ 43,035	$ 39,608	$ 36,650	$ 30,488	177,983	149,781	141,620
Provision for income taxes	25,123		18,338		11,120	10,574	12,317	12,124	11,245	9,229	65,155	44,915	42,793
Net income	$ 15,728		$ 40,769		$ 27,132	$ 29,199	$ 30,718	$ 27,484	$ 25,405	$ 21,259	112,828	104,866	98,827
Parent Company [Member]
Income
Dividend from banking subsidiaries											66,800	83,500	67,000
Deposits in banks											2	4	5
Other income											8	8	92
Total Income											66,810	83,512	67,097
Expense
Compensation and employee benefits											732	543	618
Interest Expense, Subordinated Notes and Debentures											304	0	0
Other borrowings											60	0	5
Other expense											3,090	1,608	1,368
Total Expenses											4,186	2,151	1,991
Income before income tax benefit and equity in undistributed (excess distributed) earnings of subsidiaries											62,624	81,361	65,106
Provision for income taxes											(548)	(748)	(663)
Income before equity in undistributed earnings of subsidiaries											63,172	82,109	65,769
Equity in undistributed earnings of subsidiaries											49,656	22,757	33,058
Net income											$ 112,828	$ 104,866	$ 98,827

[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.