LOANS AND ALLOWANCE FOR LOAN LOSSES - Allowance for loans collectively evaluated for impairment - (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) item	Apr. 28, 2017 USD ($)	Dec. 31, 2016 USD ($)
Activity in the allowance for loan losses and recorded investment of loans and allowances
Purchased credit impaired loans	$ 9,643		$ 8,954
Number of purchased credit impaired loans | item	0
FCB Bancorp, Inc.
Activity in the allowance for loan losses and recorded investment of loans and allowances
Purchased credit impaired loans	$ 10,186	$ 10,186
Cheviot Financial Corp.
Activity in the allowance for loan losses and recorded investment of loans and allowances
Purchased credit impaired loans			16,175
Estimated Fair Value | FCB Bancorp, Inc.
Activity in the allowance for loan losses and recorded investment of loans and allowances
Purchased credit impaired loans	$ 7,614
Estimated Fair Value | Cheviot Financial Corp.
Activity in the allowance for loan losses and recorded investment of loans and allowances
Purchased credit impaired loans			$ 11,560