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, 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
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				$ 75,646						$ 70,043	$ 75,646	$ 70,043	$ 68,172
Charge-offs											(18,555)	(16,660)	(21,462)
Recoveries											11,509	13,632	12,555
Provision for loan and lease losses	$ 1,789	$ 3,153	$ 3,975	5,852	$ 3,327	[1]	$ (648)		$ 3,177	2,775	14,769	8,631	10,778
Balance at the end of year	83,369				75,646						83,369	75,646	70,043
Specific Reserve	2,132				2,360						2,132	2,360	761
General Allocation	81,237				73,286						81,237	73,286	69,282
Consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				5,163						3,534	5,163	3,534	3,531
Charge-offs											(1,194)	(1,474)	(1,238)
Recoveries											1,180	1,187	933
Provision for loan and lease losses											152	1,916	308
Balance at the end of year	5,301				5,163						5,301	5,163	3,534
Specific Reserve	31				199						31	199	57
General Allocation	5,270				4,964						5,270	4,964	3,477
Purchased Credit Impaired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				6,907						10,515	6,907	10,515	13,726
Charge-offs											(4,862)	(6,812)	(9,944)
Recoveries											3,847	6,187	7,004
Provision for loan and lease losses											(2,281)	(2,983)	(271)
Balance at the end of year	3,611				6,907						3,611	6,907	10,515
Specific Reserve	0				0						0	0	0
General Allocation	3,611				6,907						3,611	6,907	10,515
Unallocated Financing Receivables [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				0						226	0	226	569
Charge-offs											0	0	0
Recoveries											0	0	0
Provision for loan and lease losses											1,053	(226)	(343)
Balance at the end of year	1,053				0						1,053	0	226
Specific Reserve	0				0						0	0	0
General Allocation	1,053				0						1,053	0	226
Commercial business: Secured loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				29,341						36,050	29,341	36,050	32,321
Charge-offs											(11,560)	(7,524)	(9,993)
Recoveries											3,024	4,283	2,483
Provision for loan and lease losses											22,383	(3,468)	11,239
Balance at the end of year	43,188				29,341						43,188	29,341	36,050
Specific Reserve	2,023				1,867						2,023	1,867	664
General Allocation	41,165				27,474						41,165	27,474	35,386
Commercial business: Unsecured loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				2,000						960	2,000	960	1,299
Charge-offs											(159)	(89)	(75)
Recoveries											403	553	162
Provision for loan and lease losses											382	576	(426)
Balance at the end of year	2,626				2,000						2,626	2,000	960
Specific Reserve	0				3						0	3	0
General Allocation	2,626				1,997						2,626	1,997	960
Real estate: One-to-four family residential [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				701						599	701	599	916
Charge-offs											0	(460)	(35)
Recoveries											408	568	171
Provision for loan and lease losses											(516)	(6)	(453)
Balance at the end of year	593				701						593	701	599
Specific Reserve	8				103						8	103	12
General Allocation	585				598						585	598	587
Real estate: Commercial and multifamily residential: Commercial land [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				4,265						1,797	4,265	1,797	1,178
Charge-offs											0	0	(26)
Recoveries											99	53	2
Provision for loan and lease losses											(417)	2,415	643
Balance at the end of year	3,947				4,265						3,947	4,265	1,797
Specific Reserve	0				0						0	0	0
General Allocation	3,947				4,265						3,947	4,265	1,797
Real estate: Commercial and multifamiIy residential: Income property [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				5,672						7,342	5,672	7,342	6,616
Charge-offs											(780)	(287)	0
Recoveries											912	498	966
Provision for loan and lease losses											(1,760)	(1,881)	(240)
Balance at the end of year	4,044				5,672						4,044	5,672	7,342
Specific Reserve	1				185						1	185	27
General Allocation	4,043				5,487						4,043	5,487	7,315
Real estate: Commercial and multifamiIy residential: Owner occupied [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				5,459						6,439	5,459	6,439	5,550
Charge-offs											0	0	(63)
Recoveries											20	124	434
Provision for loan and lease losses											(946)	(1,104)	518
Balance at the end of year	4,533				5,459						4,533	5,459	6,439
Specific Reserve	69				3						69	3	0
General Allocation	4,464				5,456						4,464	5,456	6,439
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				963						316	963	316	339
Charge-offs											0	(14)	(88)
Recoveries											726	72	57
Provision for loan and lease losses											(1,140)	589	8
Balance at the end of year	549				963						549	963	316
Specific Reserve	0				0						0	0	1
General Allocation	549				963						549	963	315
Real estate construction: One-to-four family residential: Residential construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				3,709						669	3,709	669	733
Charge-offs											0	0	0
Recoveries											890	106	234
Provision for loan and lease losses											937	2,934	(298)
Balance at the end of year	5,536				3,709						5,536	3,709	669
Specific Reserve	0				0						0	0	0
General Allocation	5,536				3,709						5,536	3,709	669
Real estate construction: Commercial and multifamily residential: Income property [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				7,053						404	7,053	404	388
Charge-offs											0	0	0
Recoveries											0	1	109
Provision for loan and lease losses											(1,269)	6,648	(93)
Balance at the end of year	5,784				7,053						5,784	7,053	404
Specific Reserve	0				0						0	0	0
General Allocation	5,784				7,053						5,784	7,053	404
Real estate construction: Commercial and multifamily residential: Owner occupied [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year				$ 4,413						$ 1,192	4,413	1,192	1,006
Charge-offs											0	0	0
Recoveries											0	0	0
Provision for loan and lease losses											(1,809)	3,221	186
Balance at the end of year	2,604				4,413						2,604	4,413	1,192
Specific Reserve	0				0						0	0	0
General Allocation	$ 2,604				$ 4,413						$ 2,604	$ 4,413	$ 1,192

[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.