Allowance for Loan and Lease Losses and Unfunded Commitments and Letters of Credit (Allowance for Noncovered Loan and Lease Losses) (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
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						$ 70,043				$ 68,172	$ 70,043	$ 68,172	$ 69,569
Charge-offs											(16,660)	(21,462)	(25,067)
Recoveries											13,632	12,555	15,079
Provision for loan and lease losses	$ 3,327	[1]	$ (648)		$ 3,177	2,775	$ 18	$ 1,866	$ 3,640	5,254	8,631	10,778	8,591
Balance at the end of year	75,646						70,043				75,646	70,043	68,172
Specific Reserve	2,360						761				2,360	761	653
General Allocation	73,286						69,282				73,286	69,282	67,519
Consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						3,534				3,531	3,534	3,531	3,180
Charge-offs											(1,474)	(1,238)	(2,066)
Recoveries											1,187	933	931
Provision for loan and lease losses											1,916	308	1,486
Balance at the end of year	5,163						3,534				5,163	3,534	3,531
Specific Reserve	199						57				199	57	15
General Allocation	4,964						3,477				4,964	3,477	3,516
Purchased Credit Impaired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						10,515				13,726	10,515	13,726	16,336
Charge-offs											(6,812)	(9,944)	(13,854)
Recoveries											6,187	7,004	7,329
Provision for loan and lease losses											(2,983)	(271)	3,915
Balance at the end of year	6,907						10,515				6,907	10,515	13,726
Specific Reserve	0						0				0	0	0
General Allocation	6,907						10,515				6,907	10,515	13,726
Unallocated Financing Receivables [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						226				569	226	569	1,844
Charge-offs											0	0	0
Recoveries											0	0	0
Provision for loan and lease losses											(226)	(343)	(1,275)
Balance at the end of year	0						226				0	226	569
Specific Reserve	0						0				0	0	0
General Allocation	0						226				0	226	569
Commercial business: Secured loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						36,050				32,321	36,050	32,321	25,923
Charge-offs											(7,524)	(9,993)	(7,486)
Recoveries											4,283	2,483	2,069
Provision for loan and lease losses											(3,468)	11,239	11,815
Balance at the end of year	29,341						36,050				29,341	36,050	32,321
Specific Reserve	1,867						664				1,867	664	321
General Allocation	27,474						35,386				27,474	35,386	32,000
Commercial business: Unsecured loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						960				1,299	960	1,299	927
Charge-offs											(89)	(75)	(780)
Recoveries											553	162	267
Provision for loan and lease losses											576	(426)	885
Balance at the end of year	2,000						960				2,000	960	1,299
Specific Reserve	3						0				3	0	0
General Allocation	1,997						960				1,997	960	1,299
Real estate: One-to-four family residential [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						599				916	599	916	2,281
Charge-offs											(460)	(35)	(376)
Recoveries											568	171	307
Provision for loan and lease losses											(6)	(453)	(1,296)
Balance at the end of year	701						599				701	599	916
Specific Reserve	103						12				103	12	314
General Allocation	598						587				598	587	602
Real estate: Commercial and multifamily residential: Commercial land [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						1,797				1,178	1,797	1,178	799
Charge-offs											0	(26)	0
Recoveries											53	2	291
Provision for loan and lease losses											2,415	643	88
Balance at the end of year	4,265						1,797				4,265	1,797	1,178
Specific Reserve	0						0				0	0	0
General Allocation	4,265						1,797				4,265	1,797	1,178
Real estate: Commercial and multifamiIy residential: Income property [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						7,342				6,616	7,342	6,616	9,159
Charge-offs											(287)	0	(390)
Recoveries											498	966	3,568
Provision for loan and lease losses											(1,881)	(240)	(5,721)
Balance at the end of year	5,672						7,342				5,672	7,342	6,616
Specific Reserve	185						27				185	27	0
General Allocation	5,487						7,315				5,487	7,315	6,616
Real estate: Commercial and multifamiIy residential: Owner occupied [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						6,439				5,550	6,439	5,550	5,007
Charge-offs											0	(63)	(115)
Recoveries											124	434	116
Provision for loan and lease losses											(1,104)	518	542
Balance at the end of year	5,459						6,439				5,459	6,439	5,550
Specific Reserve	3						0				3	0	0
General Allocation	5,456						6,439				5,456	6,439	5,550
Real estate construction: One-to-four family residential: Land and acquisition [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						316				339	316	339	1,197
Charge-offs											(14)	(88)	0
Recoveries											72	57	103
Provision for loan and lease losses											589	8	(961)
Balance at the end of year	963						316				963	316	339
Specific Reserve	0						1				0	1	0
General Allocation	963						315				963	315	339
Real estate construction: One-to-four family residential: Residential construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						669				733	669	733	1,860
Charge-offs											0	0	0
Recoveries											106	234	90
Provision for loan and lease losses											2,934	(298)	(1,217)
Balance at the end of year	3,709						669				3,709	669	733
Specific Reserve	0						0				0	0	3
General Allocation	3,709						669				3,709	669	730
Real estate construction: Commercial and multifamily residential: Income property [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						404				388	404	388	622
Charge-offs											0	0	0
Recoveries											1	109	8
Provision for loan and lease losses											6,648	(93)	(242)
Balance at the end of year	7,053						404				7,053	404	388
Specific Reserve	0						0				0	0	0
General Allocation	7,053						404				7,053	404	388
Real estate construction: Commercial and multifamily residential: Owner occupied [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year						$ 1,192				$ 1,006	1,192	1,006	434
Charge-offs											0	0	0
Recoveries											0	0	0
Provision for loan and lease losses											3,221	186	572
Balance at the end of year	4,413						1,192				4,413	1,192	1,006
Specific Reserve	0						0				0	0	0
General Allocation	$ 4,413						$ 1,192				$ 4,413	$ 1,192	$ 1,006

[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.