Parent Company Financial Information (Condensed Statements of Cash Flows) (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		Sep. 30, 2017	[2]	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Stock Issued											$ 0	$ 636,385	$ 0
Operating Activities
Net income	$ 44,748	$ 46,415	$ 41,749	$ 39,970	$ 15,728	[1]	$ 40,769		$ 27,132	$ 29,199	172,882	112,828	104,866
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense											8,354	7,745	5,009
Net cash provided by operating activities											237,201	128,525	145,847
Net cash received in business combinations											0	80,472	0
Investing Activities
Net cash provided by investing activities											(506,083)	(199,489)	(548,553)
Financing Activities
Payments of Ordinary Dividends, Preferred Stock and Preference Stock											0	0	(157)
Cash dividends paid											(83,440)	(51,308)	(88,677)
Repayments of Subordinated Debt											8,248	6,186	0
Payments for Repurchase of Equity											0	(7,345)	0
Purchase and retirement of common stock											(2,677)	(2,299)	(1,125)
Proceeds from exercise of stock options											1,857	1,980	1,349
Excess tax benefit from stock-based compensation											0	0	344
Net cash provided by financing activities											203,936	189,259	451,642
Increase (decrease) in cash and cash equivalents											(64,946)	118,295	48,936
Cash and cash equivalents at beginning of period				342,533						224,238	342,533	224,238	175,302
Cash and cash equivalents at end of period	277,587				342,533						277,587	342,533	224,238
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Stock Issued											0	636,385	0
Operating Activities
Net income											172,882	112,828	104,866
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed earnings of subsidiaries											(91,458)	(49,656)	(22,757)
Stock-based compensation expense											8,354	7,745	5,009
Net changes in other assets and liabilities											1,622	1,672	(394)
Net cash provided by operating activities											91,400	72,589	86,724
Net cash received in business combinations											0	(580)	0
Investing Activities
Net cash provided by investing activities											0	(580)	0
Financing Activities
Payments of Ordinary Dividends, Preferred Stock and Preference Stock											0	0	(157)
Cash dividends paid											(83,459)	(51,308)	(88,677)
Repayments of Subordinated Debt											8,248	6,186	0
Payments for Repurchase of Equity											0	7,345	0
Purchase and retirement of common stock											(2,677)	(2,299)	(1,125)
Proceeds from exercise of stock options											1,857	1,980	1,349
Excess tax benefit from stock-based compensation											0	0	344
Net cash provided by financing activities											(92,527)	(65,158)	(88,266)
Increase (decrease) in cash and cash equivalents											(1,127)	6,851	(1,542)
Cash and cash equivalents at beginning of period				$ 9,298						$ 2,447	9,298	2,447	3,989
Cash and cash equivalents at end of period	$ 8,171				$ 9,298						$ 8,171	$ 9,298	$ 2,447

[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.