Business Combinations (Tables) - Pacific Continental [Member]

12 Months Ended

Dec. 31, 2017

Business Acquisition [Line Items]

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block]

The table below summarizes the amounts recognized as of the acquisition date for each major class of assets acquired and liabilities assumed:

	November 1, 2017
	(in thousands)
Merger consideration				$	637,103
Identifiable net assets acquired, at fair value
Assets acquired
Cash and cash equivalents	$	81,190
Investment Securities	449,291
Federal Home Loan Bank stock	7,084
Loans	1,873,987
Interest receivable	7,827
Premises and equipment	27,343
Other real estate owned	10,279
Core deposit intangible	46,875
Other assets	50,638
Total assets acquired				2,554,514
Liabilities assumed
Deposits	(2,118,982		)
Federal Home Loan Bank advances	(101,127		)
Subordinated debentures	(35,678		)
Junior subordinated debentures	(14,434		)
Securities sold under agreements to repurchase	(1,617		)
Other liabilities	(28,653		)
Total liabilities assumed				(2,300,491		)
Total fair value of identifiable net assets, at fair value				254,023
Goodwill				$	383,080

Business Acquisition, Pro Forma Information [Table Text Block]

For illustrative purposes only, the following table presents certain unaudited pro forma information for the years ended December 31, 2017 and 2016. This unaudited estimated pro forma financial information was calculated as if Pacific Continental had been acquired as of the beginning of the year prior to the date of acquisition. This unaudited pro forma information combines the historical results of Pacific Continental with the Company’s consolidated historical results and includes certain adjustments reflecting the estimated impact of certain fair value adjustments for the respective periods. The pro forma information is not indicative of what would have occurred had the acquisition occurred as of the beginning of the year prior to the acquisition. In particular, no adjustments have been made to eliminate the impact of other-than-temporary impairment losses and losses recognized on the sale of securities that may not have been necessary had the investment securities been recorded at fair value as of the beginning of the year prior to the date of acquisition. The unaudited pro forma information does not consider any changes to the provision for credit losses resulting from recording loan assets at fair value. Additionally, Columbia expects to achieve further operating cost savings and other business synergies, including revenue growth as a result of the acquisition, which are not reflected in the pro forma amounts that follow. As a result, actual amounts would have differed from the unaudited pro forma information presented.

	Unaudited Pro Forma
	Years Ended December 31,
	2017		2016
	(in thousands except per share)
Total revenues (net interest income plus noninterest income)	$	571,944	$	520,419
Net income	$	149,859	$	124,550
Earnings per share - basic	$	2.23	$	1.86
Earnings per share - diluted	$	2.23	$	1.86

Business Acquisition, Integration, Restructuring and Other Related Costs [Text Block]

The following table shows the impact of the acquisition-related expenses related to the acquisition of Pacific Continental for the periods indicated to the various components of noninterest expense:

	Year ended December 31,
	2017		2016
	(in thousands)
Noninterest Expense
Compensation and employee benefits	$	8,014	$	—
Occupancy	1,912		—
Advertising and promotion	467		—
Data processing	1,555		—
Legal and professional fees	4,618		291
Taxes, licenses and fees	10		—
Other	620		—
Total impact of acquisition-related costs to noninterest expense	$	17,196	$	291