FAIR VALUE MEASUREMENT AND FAIR VALUE OF FINANCIAL INSTRUMENTS (Details 6) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financial assets:
Cash and cash equivalents	$ 1,878,503	$ 1,360,887	$ 1,039,885	$ 895,820
Interest-bearing deposits with banks	323,148	299,916
Resale agreements	2,000,000	1,600,000
Held-to-maturity investment security	143,971	0
Loans held-for-sale	23,076	31,958
Loans held-for-investment, net	25,242,619	23,378,789
Investment in FHLB stock	17,250	28,770
Investment in Federal Reserve Bank stock	55,525	54,932
Customer deposits:
Demand, interest checking, saving and money market deposits	24,275,714	20,859,086
Time deposits	5,615,269	6,616,895
Short-term borrowings	60,050	0
FHLB advances	321,643	1,019,424
Repurchase agreements	350,000	0
Long-term debt	186,327	206,084
Carrying amount of repurchase agreements eligible for netting against resale agreements	100,000	450,000
Gross repurchase agreements	450,000	450,000
Carrying Amount
Financial assets:
Cash and cash equivalents	1,878,503	1,360,887
Interest-bearing deposits with banks	323,148	299,916
Resale agreements	2,000,000	1,600,000
Held-to-maturity investment security	143,971
Loans held-for-sale	23,076	31,958
Loans held-for-investment, net	25,242,619	23,378,789
Investment in FHLB stock	17,250	28,770
Investment in Federal Reserve Bank stock	55,525	54,932
Accrued interest receivable	100,524	89,243
Customer deposits:
Demand, interest checking, saving and money market deposits	24,275,714	20,859,086
Time deposits	5,615,269	6,616,895
Short-term borrowings	60,050
FHLB advances	321,643	1,019,424
Repurchase agreements	350,000
Long-term debt	186,327	206,084
Accrued interest payable	9,440	8,848
Fair Value Measurements
Financial assets:
Cash and cash equivalents	1,878,503	1,360,887
Interest-bearing deposits with banks	323,148	299,916
Resale agreements	1,980,457	1,533,961
Held-to-maturity investment security	144,593
Loans held-for-sale	23,076	31,958
Loans held-for-investment, net	24,915,143	23,000,817
Investment in FHLB stock	17,250	28,770
Investment in Federal Reserve Bank stock	55,525	54,932
Accrued interest receivable	100,524	89,243
Customer deposits:
Demand, interest checking, saving and money market deposits	24,275,714	20,859,086
Time deposits	5,611,746	6,606,942
Short-term borrowings	60,050
FHLB advances	334,859	1,032,000
Repurchase agreements	411,368
Long-term debt	186,670	186,593
Accrued interest payable	9,440	8,848
Fair Value Measurements | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	1,878,503	1,360,887
Fair Value Measurements | Significant Other Observable Inputs (Level 2)
Financial assets:
Interest-bearing deposits with banks	323,148	299,916
Resale agreements	1,980,457	1,533,961
Loans held-for-sale	23,076	31,958
Investment in FHLB stock	17,250	28,770
Investment in Federal Reserve Bank stock	55,525	54,932
Accrued interest receivable	100,524	89,243
Customer deposits:
Demand, interest checking, saving and money market deposits	24,275,714	20,859,086
Time deposits	5,611,746	6,606,942
Short-term borrowings	60,050
FHLB advances	334,859	1,032,000
Repurchase agreements	411,368
Long-term debt	186,670	186,593
Accrued interest payable	9,440	8,848
Fair Value Measurements | Significant Unobservable Inputs (Level 3)
Financial assets:
Held-to-maturity investment security	144,593
Loans held-for-investment, net	$ 24,915,143	$ 23,000,817