Parent Company Financial Information (Condensed Statements of Cash Flows) (Details) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2017		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
Condensed Financial Statements, Captions [Line Items]
Stock Issued											$ 636,385	$ 0	$ 0
Operating Activities
Net income	$ 15,728	[1]	$ 40,769		$ 27,132	$ 29,199	$ 30,718	$ 27,484	$ 25,405	$ 21,259	112,828	104,866	98,827
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense											7,745	5,009	4,090
Net cash provided by operating activities											128,525	145,847	134,756
Net cash received in business combinations											80,472	0	0
Investing Activities
Net cash provided by investing activities											(199,489)	(548,553)	(423,054)
Financing Activities
Payments of Ordinary Dividends, Preferred Stock and Preference Stock											0	(157)	(137)
Cash dividends paid											(51,308)	(88,677)	(77,263)
Repayments of Subordinated Debt											6,186	0	8,248
Payments for Repurchase of Equity											(7,345)	0	0
Purchase and retirement of common stock											(2,299)	(1,125)	(906)
Proceeds from exercise of stock options											1,980	1,349	1,258
Excess tax benefit from stock-based compensation											0	344	0
Net cash provided by financing activities											189,259	451,642	275,430
Increase (decrease) in cash and cash equivalents											118,295	48,936	(12,868)
Cash and cash equivalents at beginning of period						224,238				175,302	224,238	175,302	188,170
Cash and cash equivalents at end of period	342,533						224,238				342,533	224,238	175,302
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Stock Issued											636,385	0	0
Operating Activities
Net income											112,828	104,866	98,827
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed earnings of subsidiaries											(49,656)	(22,757)	(33,058)
Stock-based compensation expense											7,745	5,009	4,090
Net changes in other assets and liabilities											1,672	(394)	(3,170)
Net cash provided by operating activities											72,589	86,724	66,689
Net cash received in business combinations											(580)	0	0
Investing Activities
Net cash provided by investing activities											(580)	0	0
Financing Activities
Payments of Ordinary Dividends, Preferred Stock and Preference Stock											0	(157)	(137)
Cash dividends paid											(51,308)	(88,677)	(77,263)
Repayments of Subordinated Debt											6,186	0	8,248
Payments for Repurchase of Equity											(7,345)	0	0
Purchase and retirement of common stock											(2,299)	(1,125)	(906)
Proceeds from exercise of stock options											1,980	1,349	1,258
Excess tax benefit from stock-based compensation											0	344	0
Net cash provided by financing activities											(65,158)	(88,266)	(85,296)
Increase (decrease) in cash and cash equivalents											6,851	(1,542)	(18,607)
Cash and cash equivalents at beginning of period						$ 2,447				$ 3,989	2,447	3,989	22,596
Cash and cash equivalents at end of period	$ 9,298						$ 2,447				$ 9,298	$ 2,447	$ 3,989

[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.