SECURED BORROWINGS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Feb. 26, 2016
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Total collateral carrying amount	$ 13,540,000	$ 13,480,000
Effective interest rate	0.66%	0.52%
Average borrowings for the effects of related derivatives held as cash flow hedges	$ 13,047,509	$ 12,651,061
Interest paid on secured borrowings	$ 79,100	$ 59,700	$ 71,100
Year-End Borrowing Rates Adjusted for Effects of Related Derivatives Held as Cash Flow Hedges [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Effective interest rate	0.85%	0.58%
FHLB Advances [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount for FHLB advances	$ 2,956,908
Accrued Interest Receivable	11,422
Borrowings outstanding for FHLB advance	$ 2,875,000
Effective interest rate	0.43%
Receivables and other assets	$ 60,000
FHLB Advances [Member] | Subsequent Event [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Borrowings outstanding for FHLB advance				$ 750,000
Agency Securities [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	10,579,208	$ 13,483,572
Accrued Interest Receivable	21,884	30,933
Borrowings outstanding	10,083,394	$ 12,806,843
Effective interest rate		0.38%
Agency Securities [Member] | Borrowings with Maturities of 30 Days or Less [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	9,080,363	$ 10,401,080
Accrued Interest Receivable	18,504	24,045
Borrowings outstanding	$ 8,585,336	$ 9,878,889
Effective interest rate	0.67%	0.35%
Agency Securities [Member] | Borrowings with Maturities of 31 to 90 Days [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	$ 423,710	$ 1,205,570
Accrued Interest Receivable	861	2,248
Borrowings outstanding	$ 346,177	$ 1,150,924
Effective interest rate	0.63%	0.35%
Agency Securities [Member] | Borrowings with Maturities Greater than 90 Days [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	$ 1,073,254	$ 1,874,892
Accrued Interest Receivable	2,519	4,640
Borrowings outstanding	$ 1,150,000	$ 1,775,000
Effective interest rate	0.75%	0.56%
Agency Securities [Member] | FHLB Advances [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Total collateral carrying amount	$ 13,536,116
Accrued Interest Receivable	33,306
Borrowings outstanding	$ 12,958,394
Effective interest rate	0.62%
Collateral for Structured Financings [Member] | Similar Borrowings [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	$ 1,881	$ 2,030
Accrued Interest Receivable	0	0
Borrowings outstanding	$ 1,881	$ 2,030
Effective interest rate	8.12%	8.11%