BASIS OF PRESENTATION AND ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	Sep. 30, 2019 USD ($) branch	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of branches operated | branch	172
Reserve requirement	$ 102,500		$ 61,200
Initial recognition of operating lease right-of-use assets	39,611
Initial recognition of operating lease liabilities	$ 42,050
Cumulative effect of change in accounting principle		$ 276		$ (28)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Initial recognition of operating lease right-of-use assets		27,300
Initial recognition of operating lease liabilities		29,700
Cumulative effect of change in accounting principle		$ 276