FAIR VALUE MEASUREMENT - Fair Value Measurements, Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		$ 1,989,436
Securities available for sale, at estimated fair value			$ 1,538,755
Derivative assets: Interest rate swaps		1,087	612
Derivative liabilities: Interest rate swaps		1,119	92
U.S. Government Agency Debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value			108,869
State and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		716,601
Securities available for sale, at estimated fair value			392,664
Other Stocks and Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		2,709
Securities available for sale, at estimated fair value			5,055
Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		732,972
Securities available for sale, at estimated fair value			718,029
Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		537,154
Securities available for sale, at estimated fair value			308,218
Equity Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value			5,920
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	[1]	5,791
Total asset fair value measurements		2,006,821	1,547,381
Total liability fair value measurements		3,320	726
Fair Value, Measurements, Recurring [Member] | U.S. Government Agency Debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value			108,869
Fair Value, Measurements, Recurring [Member] | State and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		716,601
Securities available for sale, at estimated fair value			392,664
Fair Value, Measurements, Recurring [Member] | Other Stocks and Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		2,709
Securities available for sale, at estimated fair value			5,055
Fair Value, Measurements, Recurring [Member] | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	732,972
Securities available for sale, at estimated fair value	[2]		718,029
Fair Value, Measurements, Recurring [Member] | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	537,154
Securities available for sale, at estimated fair value	[2]		308,218
Fair Value, Measurements, Recurring [Member] | Equity Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[1]		5,920
Fair Value, Measurements, Recurring [Member] | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets: Interest rate swaps		11,594	8,626
Derivative liabilities: Interest rate swaps		3,320	726
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	[1]	5,791
Total asset fair value measurements		5,791	5,920
Total liability fair value measurements		0	0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government Agency Debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value			0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | State and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0
Securities available for sale, at estimated fair value			0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Stocks and Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0
Securities available for sale, at estimated fair value			0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	0
Securities available for sale, at estimated fair value	[2]		0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	0
Securities available for sale, at estimated fair value	[2]		0
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[1]		5,920
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets: Interest rate swaps		0	0
Derivative liabilities: Interest rate swaps		0	0
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	[1]	0
Total asset fair value measurements		2,001,030	1,541,461
Total liability fair value measurements		3,320	726
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | U.S. Government Agency Debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value			108,869
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | State and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		716,601
Securities available for sale, at estimated fair value			392,664
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Other Stocks and Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		2,709
Securities available for sale, at estimated fair value			5,055
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	732,972
Securities available for sale, at estimated fair value	[2]		718,029
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	537,154
Securities available for sale, at estimated fair value	[2]		308,218
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Equity Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[1]		0
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets: Interest rate swaps		11,594	8,626
Derivative liabilities: Interest rate swaps		3,320	726
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	[1]	0
Total asset fair value measurements		0	0
Total liability fair value measurements		0	0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | U.S. Government Agency Debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value			0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | State and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0
Securities available for sale, at estimated fair value			0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Other Stocks and Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0
Securities available for sale, at estimated fair value			0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	0
Securities available for sale, at estimated fair value	[2]		0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	0
Securities available for sale, at estimated fair value	[2]		0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Equity Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[1]		0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets: Interest rate swaps		0	0
Derivative liabilities: Interest rate swaps		0	0
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		1,206	1,767
Impaired loans	[3]	37,813	6,536
Total asset fair value measurements		39,019	8,303
Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		0	0
Impaired loans	[3]	0	0
Total asset fair value measurements		0	0
Fair Value, Measurements, Nonrecurring [Member] | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		0	0
Impaired loans	[3]	0	0
Total asset fair value measurements		0	0
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		1,206	1,767
Impaired loans	[3]	37,813	6,536
Total asset fair value measurements		$ 39,019	$ 8,303

[1] With the adoption of ASU 2016-01 on January 1, 2018, these investments are included in equity investments on our consolidated balance sheets. The guidance was applied on a prospective approach resulting in prior-periods no longer being comparable. See “Note 1 - Summary of Significant Accounting and Reporting Policies” for further information.

[2] All mortgage-backed securities are issued and/or guaranteed by U.S. government agencies or U.S. government-sponsored enterprises.

[3] Impaired loans represent collateral-dependent loans with a specific valuation allowance. Losses on these loans represent charge-offs which are netted against the allowance for loan losses.