REPURCHASE ARRANGEMENTS AND SIMILAR BORROWINGS - Collateral and Rates (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	$ 13,220,000		$ 13,120,000
Average borrowings adjusted amount	12,465,629		12,510,701
Average borrowings rates adjusted for effects related cash flow derivatives (in hundredths)	0.49%		0.49%
Minimum [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Maturity period of repurchase arrangements	P30D
Maximum [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Maturity period of repurchase arrangements	P90D
Quarter-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)	0.48%		0.49%
Agency Securities [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	13,220,446		13,123,757
Accrued Interest Receivable	28,914		30,263
Borrowings Outstanding	12,590,255		12,482,900
Average Borrowing Rates (in hundredths)	0.32%		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	12,082,363		12,169,534
Accrued Interest Receivable	26,211		28,195
Borrowings Outstanding	11,506,451		11,578,211
Average Borrowing Rates (in hundredths)	0.31%		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	550,569		951,966
Accrued Interest Receivable	1,400		2,068
Borrowings Outstanding	524,261		902,432
Average Borrowing Rates (in hundredths)	0.33%		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	585,291
Accrued Interest Receivable	1,303
Borrowings Outstanding	557,320
Average Borrowing Rates (in hundredths)	0.46%
Agency Securities [Member] | Similar Borrowings [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	2,223	[1]	2,257	[1]
Accrued Interest Receivable	0	[1]	0	[1]
Borrowings Outstanding	$ 2,223	[1]	$ 2,257	[1]
Average Borrowing Rates (in hundredths)	8.09%	[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.