FAIR VALUE MEASUREMENT AND FAIR VALUE OF FINANCIAL INSTRUMENTS (Details 6) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Financial assets:
Cash and cash equivalents	$ 2,265,297	$ 1,360,887	$ 1,886,199	$ 1,039,885
Short-term investments	305,548	299,916
Resale agreements	1,800,000	1,600,000
Held-to-maturity investment security, at cost (fair value of $160,135 in 2016)	160,135	0
Loans held for sale	28,795	31,958
Loans receivable, net	23,494,126	23,378,789
Investment in FHLB stock	18,157	28,770
Investment in Federal Reserve Bank stock	55,079	54,932
Customer deposit accounts:
Short-term borrowings	10,093	0
FHLB advances	319,973	1,019,424
Repurchase agreements	200,000	0
Long-term debt	201,102	206,084
Gross repurchase agreements	450,000	450,000
Carrying amount of repurchase agreements eligible for netting against resale agreements	250,000	450,000
Carrying Amount
Financial assets:
Cash and cash equivalents	2,265,297	1,360,887
Short-term investments	305,548	299,916
Resale agreements	1,800,000	1,600,000
Held-to-maturity investment security, at cost (fair value of $160,135 in 2016)	160,135
Loans held for sale	28,795	31,958
Loans receivable, net	23,494,126	23,378,789
Investment in FHLB stock	18,157	28,770
Investment in Federal Reserve Bank stock	55,079	54,932
Accrued interest receivable	88,804	89,243
Customer deposit accounts:
Demand, savings and money market deposits	22,531,531	20,859,086
Time deposits	6,064,716	6,616,895
Short-term borrowings	10,093
FHLB advances	319,973	1,019,424
Repurchase agreements	200,000
Accrued interest payable	8,507	8,848
Long-term debt	201,102	206,084
Fair Value Measurements
Financial assets:
Cash and cash equivalents	2,265,297	1,360,887
Short-term investments	305,548	299,916
Resale agreements	1,571,665	1,533,961
Held-to-maturity investment security, at cost (fair value of $160,135 in 2016)	160,135
Loans held for sale	28,795	31,958
Loans receivable, net	23,344,433	23,000,817
Investment in FHLB stock	18,157	28,770
Investment in Federal Reserve Bank stock	55,079	54,932
Accrued interest receivable	88,804	89,243
Customer deposit accounts:
Demand, savings and money market deposits	22,531,531	20,859,086
Time deposits	6,063,265	6,606,942
Short-term borrowings	10,093
FHLB advances	332,474	1,032,000
Repurchase agreements	271,367
Accrued interest payable	8,507	8,848
Long-term debt	194,189	186,593
Fair Value Measurements | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	2,265,297	1,360,887
Fair Value Measurements | Significant Other Observable Inputs (Level 2)
Financial assets:
Short-term investments	305,548	299,916
Resale agreements	1,571,665	1,533,961
Loans held for sale	28,795	31,958
Investment in FHLB stock	18,157	28,770
Investment in Federal Reserve Bank stock	55,079	54,932
Accrued interest receivable	88,804	89,243
Customer deposit accounts:
Demand, savings and money market deposits	22,531,531	20,859,086
Time deposits	6,063,265	6,606,942
Short-term borrowings	10,093
FHLB advances	332,474	1,032,000
Repurchase agreements	271,367
Accrued interest payable	8,507	8,848
Long-term debt	194,189	186,593
Fair Value Measurements | Significant Unobservable Inputs (Level 3)
Financial assets:
Held-to-maturity investment security, at cost (fair value of $160,135 in 2016)	160,135
Loans receivable, net	$ 23,344,433	$ 23,000,817