FAIR VALUE MEASURES - Significant Unobservable Inputs Used in Fair Value Measurement of Level 3 Assets and Liabilities (Details) $ in Thousands	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment securities available for sale, at fair value	$ 1,162,570	$ 810,873
Impaired loans, at fair value	8,470,220	5,992,880
Other real estate owned, at fair value	9,375	8,464	$ 9,391	$ 10,874
Purchased other real estate owned, at fair value	7,692	9,011
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	31,182	27,684
Other real estate owned, at fair value	2,734	323
Purchased other real estate owned, at fair value	$ 7,692	$ 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 Daily Balance | 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 | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans, measurement input	0.90	0.90
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	0.24
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	0.20
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	0.54	0.15
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	0.23	0.15
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 | 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 Daily Balance | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Purchased other real estate owned, measurement input	0.38	0.26
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