FAIR VALUE MEASUREMENTS - Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	$ 1,575,289	$ 1,311,555
Marketable equity securities	31,993	41,556
Total Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading security	9,525	10,769
Securities available for sale	1,575,289	1,311,555
Marketable equity securities	31,993	41,556
Loans held for investment at fair value	9,172,009	9,653,550
Loans held for sale	17,596	169,319
Derivative assets	179,626	80,190
Derivative liabilities	73,659	80,681
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading security	0	0
Securities available for sale	0	0
Marketable equity securities	31,321	40,499
Loans held for investment at fair value	0	0
Loans held for sale	0	0
Derivative assets	0	0
Derivative liabilities	0	227
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading security	0	0
Securities available for sale	1,549,464	1,267,573
Marketable equity securities	672	1,057
Loans held for investment at fair value	0	0
Loans held for sale	17,596	140,280
Derivative assets	177,731	77,562
Derivative liabilities	73,659	80,454
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading security	9,525	10,769
Securities available for sale	25,825	43,982
Marketable equity securities	0	0
Loans held for investment at fair value	9,172,009	9,653,550
Loans held for sale	0	29,039
Derivative assets	1,895	2,628
Derivative liabilities	0	0
Recurring | Total Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading security	9,525	10,769
Loans held for sale	17,596	140,280
Derivative assets	179,626	80,190
Capitalized servicing rights	3,855	12,229
Derivative liabilities	73,659	80,681
Recurring | Total Fair Value | Debt securities | Municipal bonds and obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	104,507	110,138
Recurring | Total Fair Value | Debt securities | Agency collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	741,469	748,812
Recurring | Total Fair Value | Debt securities | Agency residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	326,017	147,744
Recurring | Total Fair Value | Debt securities | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	273,608	147,096
Recurring | Total Fair Value | Debt securities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	74,977	116,576
Recurring | Total Fair Value | Debt securities | Other bonds and obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	54,711	41,189
Recurring | Total Fair Value | Marketable equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	31,993	41,556
Loans held for investment at fair value	2,774	0
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading security	0	0
Loans held for sale	0	0
Derivative assets	0	0
Capitalized servicing rights	0	0
Derivative liabilities	0	227
Recurring | Level 1 | Debt securities | Municipal bonds and obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Recurring | Level 1 | Debt securities | Agency collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Recurring | Level 1 | Debt securities | Agency residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Recurring | Level 1 | Debt securities | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Recurring | Level 1 | Debt securities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Recurring | Level 1 | Debt securities | Other bonds and obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Recurring | Level 1 | Marketable equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	31,321	40,499
Loans held for investment at fair value	0	0
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading security	0	0
Loans held for sale	17,596	140,280
Derivative assets	177,731	77,562
Capitalized servicing rights	0	0
Derivative liabilities	73,659	80,454
Recurring | Level 2 | Debt securities | Municipal bonds and obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	104,507	110,138
Recurring | Level 2 | Debt securities | Agency collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	741,469	748,812
Recurring | Level 2 | Debt securities | Agency residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	326,017	147,744
Recurring | Level 2 | Debt securities | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	273,608	147,096
Recurring | Level 2 | Debt securities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	49,152	73,610
Recurring | Level 2 | Debt securities | Other bonds and obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	54,711	41,189
Recurring | Level 2 | Marketable equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	672	1,057
Loans held for investment at fair value	0	0
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading security	9,525	10,769
Securities available for sale	25,825	42,966
Loans held for sale	0	0
Derivative assets	1,895	2,628
Capitalized servicing rights	3,855	12,229
Derivative liabilities	0	0
Recurring | Level 3 | Debt securities | Municipal bonds and obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Recurring | Level 3 | Debt securities | Agency collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Recurring | Level 3 | Debt securities | Agency residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Recurring | Level 3 | Debt securities | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Recurring | Level 3 | Debt securities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	25,825	42,966
Recurring | Level 3 | Debt securities | Other bonds and obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Recurring | Level 3 | Marketable equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	0	0
Loans held for investment at fair value	$ 2,774	$ 0