Parent Company Financial Information (Condensed Statements of 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	[1]	Sep. 30, 2017	[2]	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income
Deposits in banks											$ 702	$ 813	$ 216
Expense
Compensation and employee benefits											200,199	169,674	150,282
Interest Expense, Subordinated Notes and Debentures											1,871	304	0
Other borrowings											504	575	545
Other expense											36,627	34,161	28,362
Income before income tax benefit and equity in undistributed (excess distributed) earnings of subsidiaries	$ 55,482	$ 57,821	$ 51,748	$ 46,785	$ 40,851		$ 59,107		$ 38,252	$ 39,773	211,836	177,983	149,781
Provision for income taxes	10,734	11,406	9,999	6,815	25,123		18,338		11,120	10,574	38,954	65,155	44,915
Net income	$ 44,748	$ 46,415	$ 41,749	$ 39,970	$ 15,728		$ 40,769		$ 27,132	$ 29,199	172,882	112,828	104,866
Parent Company [Member]
Income
Dividend from banking subsidiaries											85,250	66,800	83,500
Deposits in banks											12	2	4
Other income											56	8	8
Total Income											85,318	66,810	83,512
Expense
Compensation and employee benefits											978	732	543
Interest Expense, Subordinated Notes and Debentures											1,871	304	0
Other borrowings											4	60	0
Other expense											2,058	3,090	1,608
Total Expenses											4,911	4,186	2,151
Income before income tax benefit and equity in undistributed (excess distributed) earnings of subsidiaries											80,407	62,624	81,361
Provision for income taxes											(1,017)	(548)	(748)
Income before equity in undistributed earnings of subsidiaries											81,424	63,172	82,109
Equity in undistributed earnings of subsidiaries											91,458	49,656	22,757
Net income											$ 172,882	$ 112,828	$ 104,866

[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.