Summary Of Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Total interest income	$ 108,841		$ 90,303		$ 87,786		$ 87,816		$ 86,734		$ 86,758		$ 83,303		$ 81,174		$ 374,746		$ 337,969		$ 328,891
Total interest expense	2,617		1,374		1,625		1,141		997		1,186		1,163		1,004		6,757		4,350		4,004
Net Interest Income	106,224		88,929		86,161		86,675		85,737		85,572		82,140		80,170		367,989		333,619		324,887
Provision for loan and lease losses	3,327		(648)		3,177		2,775		18		1,866		3,640		5,254		8,631		10,778		8,591
Noninterest income	23,581		37,067		24,135		24,859		22,330		23,166		21,940		20,646		109,642		88,082		91,473
Noninterest expense	85,627		67,537		68,867		68,986		65,014		67,264		63,790		65,074		291,017		261,142		266,149
Income before income taxes	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
Per Common Share
Earnings (basic)	$ 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 (diluted)	$ 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

[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.