FAIR VALUE MEASURES - Summary of Significant Unobservable Inputs Used in Fair Value Measurement of Level 3 Assets and Liabilities (Details) $ in Thousands	Dec. 31, 2018 USD ($)	Sep. 30, 2018	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale, at fair value	$ 1,192,423		$ 810,873
Other real estate owned	9,535		9,011
Purchased other real estate owned	7,218		8,464	$ 10,874
Fair Value, Measurements, Recurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale, at fair value	1,500		1,500
Fair Value, Measurements, Nonrecurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total impaired loans	28,653		27,684
Other real estate owned	408		323
Purchased other real estate owned	$ 9,535		$ 9,011
Discounted par values | Credit quality of underlying issuer | Fair Value, Measurements, Recurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale, measurement input	0.00		0.00
Discounted par values | Credit quality of underlying issuer | Fair Value, Measurements, Recurring | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale, measurement input	0.00		0.00
Third party appraisals and discounted cash flows | Collateral discounts and discount rates | Fair Value, Measurements, Nonrecurring | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans, measurement input		0.03	0.20
Third party appraisals and discounted cash flows | Collateral discounts and discount rates | Fair Value, Measurements, Nonrecurring | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans, measurement input		0.53	0.90
Third party appraisals and discounted cash flows | Collateral discounts and discount rates | Fair Value, Measurements, Nonrecurring | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans, measurement input	0.30		0.24
Third party appraisals and sales contracts | Collateral discounts and estimated costs to sell | Fair Value, Measurements, Nonrecurring | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other real estate owned, measurement input		0.15	0.15
Third party appraisals and sales contracts | Collateral discounts and estimated costs to sell | Fair Value, Measurements, Nonrecurring | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other real estate owned, measurement input		0.69	0.15
Third party appraisals and sales contracts | Collateral discounts and estimated costs to sell | Fair Value, Measurements, Nonrecurring | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other real estate owned, measurement input	0.31		0.15
Third party appraisals | Collateral discounts and estimated costs to sell | Fair Value, Measurements, Nonrecurring | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Purchased other real estate owned, measurement input		0.06	0.10
Third party appraisals | Collateral discounts and estimated costs to sell | Fair Value, Measurements, Nonrecurring | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Purchased other real estate owned, measurement input		0.74	0.74
Third party appraisals | Collateral discounts and estimated costs to sell | Fair Value, Measurements, Nonrecurring | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Purchased other real estate owned, measurement input	0.39		0.26