LOANS AND ALLOWANCE FOR LOAN LOSSES - Credit Risk Profile (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Financing Receivable, Recorded Investment [Line Items]
Loan commitments rated substandard or worse that are reviewed for loss potential	$ 500,000
Commercial	5,621,336,000	5,020,565,000
Mortgage finance	3,774,467,000	2,784,265,000
Construction	1,640,596,000	1,262,905,000
Real estate	2,362,230,000	2,146,228,000
Consumer	16,502,000	15,350,000
Leases	101,327,000	93,160,000
Gross loans held for investment	13,516,458,000	11,322,473,000	10,362,840,000
Pass
Financing Receivable, Recorded Investment [Line Items]
Commercial	5,479,240,000	4,908,944,000
Mortgage finance	3,774,467,000	2,784,265,000
Construction	1,640,596,000	1,261,995,000
Real estate	2,332,791,000	2,099,450,000
Consumer	16,384,000	15,251,000
Leases	96,269,000	89,317,000
Gross loans held for investment	13,339,747,000	11,159,222,000
Special mention
Financing Receivable, Recorded Investment [Line Items]
Commercial	68,215,000	24,132,000
Mortgage finance	0	0
Construction	0	102,000
Real estate	7,490,000	6,338,000
Consumer	13,000	0
Leases	777,000	51,000
Gross loans held for investment	76,495,000	30,623,000
Substandard-accruing
Financing Receivable, Recorded Investment [Line Items]
Commercial	48,875,000	74,593,000
Mortgage finance	0	0
Construction	0	103,000
Real estate	9,232,000	21,770,000
Consumer	105,000	45,000
Leases	4,271,000	3,742,000
Gross loans held for investment	62,483,000	100,253,000
Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Commercial	25,006,000	12,896,000
Mortgage finance	0	0
Construction	0	705,000
Real estate	12,717,000	18,670,000
Consumer	0	54,000
Leases	10,000	50,000
Gross loans held for investment	$ 37,733,000	$ 32,375,000