FAIR VALUE MEASUREMENTS - Fair Values and Carrying Amounts (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financial Assets
Marketable equity securities	$ 31,993	$ 41,556
Securities available for sale	1,575,289	1,311,555
Securities held to maturity	356,035	373,277
FHLB bank stock and restricted securities	40,520	48,019
Level 1
Financial Assets
Cash and cash equivalents	935,292	579,829
Trading security	0	0
Marketable equity securities	31,321	40,499
Securities available for sale	0	0
Securities held to maturity	0	0
Net loans	0	0
Loans held for sale	0	0
Accrued interest receivable	0	0
Derivative assets	0	0
Financial Liabilities
Total deposits	0	0
Short-term debt	0	0
Long-term Federal Home Loan Bank advances and other	0	0
Subordinated borrowings	0	0
Derivative liabilities	0	227
Level 2
Financial Assets
Cash and cash equivalents	0	0
Trading security	0	0
Marketable equity securities	672	1,057
Securities available for sale	1,549,464	1,267,573
Securities held to maturity	352,485	355,513
Net loans	0	0
Loans held for sale	17,596	140,280
Accrued interest receivable	48,467	36,462
Derivative assets	177,731	77,562
Financial Liabilities
Total deposits	10,483,734	10,338,993
Short-term debt	110,214	125,081
Long-term Federal Home Loan Bank advances and other	500,933	606,381
Subordinated borrowings	94,287	101,055
Derivative liabilities	73,659	80,454
Level 3
Financial Assets
Cash and cash equivalents	0	0
Trading security	9,525	10,769
Marketable equity securities	0	0
Securities available for sale	25,825	43,982
Securities held to maturity	3,550	17,764
Net loans	9,172,009	9,653,550
Loans held for sale	0	29,039
Accrued interest receivable	0	0
Derivative assets	1,895	2,628
Financial Liabilities
Total deposits	0	0
Short-term debt	0	0
Long-term Federal Home Loan Bank advances and other	0	0
Subordinated borrowings	0	0
Derivative liabilities	0	0
Carrying Amount
Financial Assets
Cash and cash equivalents	935,292	579,829
Trading security	9,525	10,769
Marketable equity securities	31,993	41,556
Securities available for sale	1,575,289	1,311,555
Securities held to maturity	330,197	357,979
FHLB bank stock and restricted securities	40,520	48,019
Net loans	8,847,922	9,438,853
Loans held for sale	17,596	169,319
Accrued interest receivable	48,467	36,462
Derivative assets	179,626	80,190
Financial Liabilities
Total deposits	10,466,559	10,335,977
Short-term debt	110,000	125,000
Long-term Federal Home Loan Bank advances and other	495,483	605,501
Subordinated borrowings	97,223	97,049
Derivative liabilities	73,659	80,681
Fair Value
Financial Assets
Cash and cash equivalents	935,292	579,829
Trading security	9,525	10,769
Marketable equity securities	31,993	41,556
Securities available for sale	1,575,289	1,311,555
Securities held to maturity	356,035	373,277
Net loans	9,172,009	9,653,550
Loans held for sale	17,596	169,319
Accrued interest receivable	48,467	36,462
Derivative assets	179,626	80,190
Financial Liabilities
Total deposits	10,483,734	10,338,993
Short-term debt	110,214	125,081
Long-term Federal Home Loan Bank advances and other	500,933	606,381
Subordinated borrowings	94,287	101,055
Derivative liabilities	$ 73,659	$ 80,681