SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Premises and Equipment, Intangible Assets, Dividends and ASU (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) segment	Jan. 01, 2019 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Accounting Policies [Line Items]
Servicing assets	$ 1,200		$ 1,200
Dividends
Period from which net retained earnings are available for dividend payment from the Bank to the Parent Company	2 years
Amount up to which dividends can be declared by the Bank to the Parent Company	$ 51,400
Cash dividends paid	15,000
Income Taxes
Unrecognized Tax Benefits	$ 0		$ 0
Segment Reporting
Number of reportable operating segment | segment	1
ASU 2016-02 | Measurement Period Adjustments
Leases
Operating Lease, Right of use assets		$ 39,000
Operating Lease, Liability		$ 39,000
Core deposit intangibles
Summary Of Accounting Policies [Line Items]
Intangible assets, useful life (in years)	10 years
Buildings
Summary Of Accounting Policies [Line Items]
Useful life	50 years
Equipment, computer hardware and software and furniture and fixtures | Minimum
Summary Of Accounting Policies [Line Items]
Useful life	2 years
Equipment, computer hardware and software and furniture and fixtures | Maximum
Summary Of Accounting Policies [Line Items]
Useful life	10 years