BUSINESS COMBINATION (Details) $ in Thousands	Nov. 01, 2019 shares	Nov. 01, 2018 USD ($) shares	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 339,154	$ 339,154	$ 242,659
Nonaccretable difference			(6,300)
AltaPacific Bank [Member]
Business Acquisition [Line Items]
Business Combination, Combined Assets			12,500,000
Assets acquired			420,000
Loans receivable (contractual amount of $645.6 million)			334,000
Deposits			$ 297,000
Skagit Bank [Member]
Business Acquisition [Line Items]
Percentage of voting interest acquired		100.00%
Number of branches acquired		11
Shares transferred as consideration (in shares) | shares		3,100,000
Total consideration paid		$ 180,038
Assets acquired		915,821
Deposits recognized		810,200
Loans receivable (contractual amount of $645.6 million)		632,374
Deposits		810,209
Core deposit intangible		16,368
Goodwill		$ 96,495
Acquired intangible asset, useful life		9 years
Unpaid principal balance on non-credit-impaired loans		$ 637,400
Fair value on non-credit-impaired loans		625,200
Discount on acquired loans		12,200
Credit related discount		7,900
Contractually required principal and interest payments		9,897
Nonaccretable difference		(1,915)
Cash flows expected to be collected		7,982
Accretable yield		(995)
Fair value of PCI loans		$ 6,987
Subsequent Event [Member] | AltaPacific Bank [Member]
Business Acquisition [Line Items]
Subsequent Event, Date	Nov. 01, 2019
Shares transferred as consideration (in shares) | shares	0.2712
Northern California [Member] | AltaPacific Bank [Member]
Business Acquisition [Line Items]
Number of branches acquired			1
Southern California [Member] | AltaPacific Bank [Member]
Business Acquisition [Line Items]
Number of branches acquired			5