FAIR VALUE MEASUREMENT AND FAIR VALUE OF FINANCIAL INSTRUMENTS (Details 6) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Financial assets:
Cash and cash equivalents	$ 1,666,832	$ 1,360,887	$ 1,875,703	$ 1,039,885
Short-term investments	307,473	299,916
Resale agreements	1,500,000	1,600,000
Held-to-maturity investment security	154,461	0
Loans held-for-sale	47,719	31,958
Loans held-for-investment, net	24,476,150	23,378,789
Investment in FHLB stock	17,250	28,770
Investment in Federal Reserve Bank stock	55,355	54,932
Customer deposits:
Short-term borrowings	36,992	0
FHLB advances	321,084	1,019,424
Repurchase agreements	200,000	0
Long-term debt	191,265	206,084
Carrying amount of repurchase agreements eligible for netting against resale agreements	250,000	450,000
Gross repurchase agreements	450,000	450,000
Carrying Amount
Financial assets:
Cash and cash equivalents	1,666,832	1,360,887
Short-term investments	307,473	299,916
Resale agreements	1,500,000	1,600,000
Held-to-maturity investment security	154,461
Loans held-for-sale	47,719	31,958
Loans held-for-investment, net	24,476,150	23,378,789
Investment in FHLB stock	17,250	28,770
Investment in Federal Reserve Bank stock	55,355	54,932
Accrued interest receivable	88,343	89,243
Customer deposits:
Demand, interest checking, savings and money market deposits	22,994,054	20,859,086
Time deposits	5,598,387	6,616,895
Short-term borrowings	36,992
FHLB advances	321,084	1,019,424
Repurchase agreements	200,000
Long-term debt	191,265	206,084
Accrued interest payable	7,516	8,848
Fair Value Measurements
Financial assets:
Cash and cash equivalents	1,666,832	1,360,887
Short-term investments	307,473	299,916
Resale agreements	1,496,965	1,533,961
Held-to-maturity investment security	154,296
Loans held-for-sale	47,719	31,958
Loans held-for-investment, net	24,349,603	23,000,817
Investment in FHLB stock	17,250	28,770
Investment in Federal Reserve Bank stock	55,355	54,932
Accrued interest receivable	88,343	89,243
Customer deposits:
Demand, interest checking, savings and money market deposits	22,994,054	20,859,086
Time deposits	5,603,591	6,606,942
Short-term borrowings	36,992
FHLB advances	333,437	1,032,000
Repurchase agreements	263,948
Long-term debt	195,801	186,593
Accrued interest payable	7,516	8,848
Fair Value Measurements | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	1,666,832	1,360,887
Fair Value Measurements | Significant Other Observable Inputs (Level 2)
Financial assets:
Short-term investments	307,473	299,916
Resale agreements	1,496,965	1,533,961
Loans held-for-sale	47,719	31,958
Investment in FHLB stock	17,250	28,770
Investment in Federal Reserve Bank stock	55,355	54,932
Accrued interest receivable	88,343	89,243
Customer deposits:
Demand, interest checking, savings and money market deposits	22,994,054	20,859,086
Time deposits	5,603,591	6,606,942
Short-term borrowings	36,992
FHLB advances	333,437	1,032,000
Repurchase agreements	263,948
Long-term debt	195,801	186,593
Accrued interest payable	7,516	8,848
Fair Value Measurements | Significant Unobservable Inputs (Level 3)
Financial assets:
Held-to-maturity investment security	154,296
Loans held-for-investment, net	$ 24,349,603	$ 23,000,817