REPURCHASE ARRANGEMENTS AND SIMILAR BORROWINGS - Collateral and Rates (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	$ 13,480,000		$ 13,120,000
Borrowings Outstanding	12,806,843		12,482,900
Average borrowings adjusted amount	12,651,061		12,702,941
Average borrowings rates adjusted for effects related cash flow derivatives (in hundredths)	0.52%		0.52%
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]
Average Borrowing Rates (in hundredths)	0.58%		0.49%
Agency Securities [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	13,483,572		13,123,757
Accrued Interest Receivable	30,933		30,263
Borrowings Outstanding	12,806,843		12,482,900
Average Borrowing Rates (in hundredths)	0.38%		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	10,401,080		12,169,534
Accrued Interest Receivable	24,045		28,195
Borrowings Outstanding	9,878,889		11,578,211
Average Borrowing Rates (in hundredths)	0.35%		0.38%
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	1,205,570		951,966
Accrued Interest Receivable	2,248		2,068
Borrowings Outstanding	1,150,924		902,432
Average Borrowing Rates (in hundredths)	0.35%		0.38%
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	1,874,892
Accrued Interest Receivable	4,640
Borrowings Outstanding	1,775,000
Average Borrowing Rates (in hundredths)	0.56%
Collateral For Structured Financings [Member] | Similar Borrowings [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	2,030	[1]	2,257	[1]
Accrued Interest Receivable	0	[1]	0	[1]
Borrowings Outstanding	$ 2,030	[1]	$ 2,257	[1]
Average Borrowing Rates (in hundredths)	8.11%	[1]	8.09%	[1]

[1] The maturity of structured financings is directly affected by prepayments on the related mortgage pass-through securities pledged as collateral and these financings are subject to redemption by the residual bondholders.