FAIR VALUE DISCLOSURES - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Financial Instruments, Liabilities, Fair Value Disclosure	$ 33,278	$ 12,687
Financial assets
Investment securities held-to-maturity	434,369	654,008
Other investments	115,757	53,140
Deposits
Noninterest-bearing	2,375,845	1,662,058
Savings	3,134,944	2,462,420
Time	1,957,574	1,317,105
Carrying value
Financial assets
Cash and short-term investments	222,277	184,624
Investment securities held-to-maturity	434,369	654,008
Other investments	115,757	53,140
Loans held for sale	13,277	11,502
Loans and leases	8,789,661	5,959,162
Interest Receivable	41,755	24,496
Deposits
Deposits	9,752,634	6,895,046
Short-term borrowings	1,294,252	814,565
Long-term debt	570,037	119,654
Interest Payable	10,517	5,104
Fair value
Financial assets
Cash and short-term investments	222,277	184,624
Investment securities held-to-maturity	422,638	653,101
Loans held for sale	13,277	11,502
Loans and leases	8,737,769	6,006,656
Interest Receivable	41,755	24,496
Deposits
Deposits	9,722,689	6,884,615
Short-term borrowings	1,294,252	814,565
Long-term debt	547,569	117,908
Interest Payable	10,517	5,104
Fair Value, Inputs, Level 1 [Member] | Fair value
Financial assets
Cash and short-term investments	222,277	184,624
Investment securities held-to-maturity	0	0
Loans held for sale	0	0
Loans and leases	0	0
Interest Receivable	0	0
Deposits
Deposits	0	0
Short-term borrowings	1,294,252	814,565
Long-term debt	0	0
Interest Payable	2,963	204
Fair Value, Inputs, Level 2 [Member] | Fair value
Financial assets
Cash and short-term investments	0	0
Investment securities held-to-maturity	422,638	653,101
Loans held for sale	13,277	11,502
Loans and leases	0	0
Interest Receivable	13,674	8,265
Deposits
Deposits	9,722,689	6,884,615
Short-term borrowings	0	0
Long-term debt	547,569	117,908
Interest Payable	7,554	4,900
Fair Value, Inputs, Level 3 [Member] | Fair value
Financial assets
Cash and short-term investments	0	0
Investment securities held-to-maturity	0	0
Loans held for sale	0	0
Loans and leases	8,737,769	6,006,656
Interest Receivable	28,081	16,231
Deposits
Deposits	0	0
Short-term borrowings	0	0
Long-term debt	0	0
Interest Payable	$ 0	$ 0