FAIR VALUE MEASURES - Significant Unobservable Inputs Used in Fair Value Measurement of Level 3 Assets and Liabilities (Details) $ in Thousands	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale, at fair value	$ 1,273,244	$ 1,192,423
Impaired loans, at fair value	8,987,091	8,454,442
Other real estate owned, at fair value	5,169	7,218	$ 8,003	$ 8,464
Purchased other real estate owned, at fair value	9,506	9,535
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]
Impaired loans, at fair value	30,986	28,653
Other real estate owned, at fair value	407	408
Purchased other real estate owned, at fair value	$ 9,506	$ 9,535
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	0
Discounted par values | Credit quality of underlying issuer | Fair Value, Measurements, Recurring | Weighted Average Daily Balance | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale, measurement input	0	0
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	0.10	0.03
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	0.92	0.53
Third-party appraisals and discounted cash flows | Collateral discounts and discount rates | Fair Value, Measurements, Nonrecurring | Weighted Average Daily Balance | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans, measurement input	0.26	0.30
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	0.65	0.69
Third-party appraisals and sales contracts | Collateral discounts and estimated costs to sell | Fair Value, Measurements, Nonrecurring | Weighted Average Daily Balance | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other real estate owned, measurement input	0.24	0.31
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	0.10	0.06
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	0.75	0.74
Third-party appraisals | Collateral discounts and estimated costs to sell | Fair Value, Measurements, Nonrecurring | Weighted Average Daily Balance | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Purchased other real estate owned, measurement input	0.33	0.39