SECURED BORROWINGS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2016
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Total collateral carrying amount	$ 13,090,000	$ 13,540,000	$ 13,090,000
Total borrowings outstanding	12,431,839	12,958,394	$ 12,431,839
Average borrowings for the effects of related derivatives held as cash flow hedges	$ 12,731,035	$ 13,160,703
Effective borrowing rate considering cash flow hedges	0.84%	0.73%	0.84%
Quarter-End Borrowing Rates Adjusted for Effects of Related Derivative Financial Instruments ("Derivatives") Held as Cash Flow Hedges [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Average borrowing rates	0.86%		0.86%
Year-End Borrowing Rates Adjusted for Effects of Related Derivatives Held as Cash Flow Hedges [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Average borrowing rates		0.85%
FHLB Advances [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Proceeds from sale of federal home loan bank stock	$ 20,000		$ 50,000
Investment in federal home loan bank stock	10,000		10,000
Collateral carrying amount for FHLB advances	259,367	$ 2,956,908	259,367
Accrued interest receivable	1,039	11,422	1,039
Borrowings outstanding for FHLB advance	$ 250,000	$ 2,875,000	$ 250,000
Average borrowing rates	0.71%	0.43%	0.71%
Agency Securities [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount	$ 12,830,689	$ 10,579,208	$ 12,830,689
Accrued interest receivable	27,929	21,884	27,929
Borrowings outstanding	12,181,839	10,083,394	12,181,839
Agency Securities [Member] | Borrowings with Maturities of 30 Days or Less [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount	11,302,826	9,080,363	11,302,826
Accrued interest receivable	24,045	18,504	24,045
Borrowings outstanding	$ 10,741,746	$ 8,585,336	$ 10,741,746
Average borrowing rates	0.71%	0.67%	0.71%
Agency Securities [Member] | Borrowings with Maturities of 31 to 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount	$ 1,365,926	$ 423,710	$ 1,365,926
Accrued interest receivable	2,881	861	2,881
Borrowings outstanding	$ 1,299,305	$ 346,177	$ 1,299,305
Average borrowing rates	0.72%	0.63%	0.72%
Agency Securities [Member] | Borrowings with Maturities Greater than 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount	$ 160,250	$ 1,073,254	$ 160,250
Accrued interest receivable	1,003	2,519	1,003
Borrowings outstanding	$ 139,101	$ 1,150,000	$ 139,101
Average borrowing rates	0.92%	0.75%	0.92%
Agency Securities [Member] | Similar Borrowings [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount	$ 1,687	$ 1,881	$ 1,687
Accrued interest receivable	0	0	0
Borrowings outstanding	$ 1,687	$ 1,881	$ 1,687
Average borrowing rates	8.16%	8.12%	8.16%
Agency Securities [Member] | FHLB Advances [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Total collateral carrying amount	$ 13,090,056	$ 13,536,116	$ 13,090,056
Accrued interest receivable	28,968	33,306	28,968
Total borrowings outstanding	$ 12,431,839	$ 12,958,394	$ 12,431,839
Average borrowing rates	0.72%	0.62%	0.72%