LOANS AND ALLOWANCE FOR LOAN LOSSES - Credit Risk Profile (Details) - USD ($)	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Recorded Investment [Line Items]
Loan commitments rated substandard or worse that are reviewed for loss potential	$ 500,000
Commercial	6,553,639,000	$ 5,869,219,000
Mortgage finance	4,312,790,000	4,102,125,000
Construction	1,864,178,000	1,416,405,000
Real estate	3,058,574,000	2,807,127,000
Consumer	24,757,000	19,699,000
Leases	118,644,000	99,495,000
Gross loans held for investment	15,932,582,000	14,314,070,000	$ 13,516,746,000
Pass
Financing Receivable, Recorded Investment [Line Items]
Commercial	6,244,334,000	5,738,474,000
Mortgage finance	4,312,790,000	4,102,125,000
Construction	1,845,094,000	1,414,671,000
Real estate	3,014,205,000	2,785,804,000
Consumer	24,497,000	19,579,000
Leases	110,295,000	91,044,000
Gross loans held for investment	15,551,215,000	14,151,697,000
Special mention
Financing Receivable, Recorded Investment [Line Items]
Commercial	113,542,000	53,839,000
Mortgage finance	0	0
Construction	1,771,000	1,734,000
Real estate	23,076,000	8,723,000
Consumer	26,000	11,000
Leases	2,650,000	4,363,000
Gross loans held for investment	141,065,000	68,670,000
Substandard-accruing
Financing Receivable, Recorded Investment [Line Items]
Commercial	115,565,000	43,784,000
Mortgage finance	0	0
Construction	564,000	0
Real estate	14,265,000	2,653,000
Consumer	234,000	47,000
Leases	0	3,915,000
Gross loans held for investment	130,628,000	50,399,000
Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Commercial	80,198,000	33,122,000
Mortgage finance	0	0
Construction	16,749,000	0
Real estate	7,028,000	9,947,000
Consumer	0	62,000
Leases	5,699,000	173,000
Gross loans held for investment	$ 109,674,000	$ 43,304,000