Repurchase Arrangements And Similar Borrowings, Including Related Hedging Activity (Schedule Of Repurchase Arrangements And Similar Borrowings) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011	Sep. 30, 2011
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	$ 13,930,000		$ 11,930,000
Borrowings Outstanding	13,250,488		11,352,444
Average Borrowing Rates	8.49%	[1]	0.37%	8.49%
Agency Securities [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	13,928,066		11,929,514
Accrued Interest Receivable	30,643		27,181
Borrowings Outstanding	13,250,488		11,352,444
Average Borrowing Rates	0.41%
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	0.56%		0.58%
Borrowings With Maturities Of 30 Days Or Less [Member] | Agency Securities [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	12,903,773		11,306,478
Accrued Interest Receivable	28,526		25,630
Borrowings Outstanding	12,279,241		10,754,835
Average Borrowing Rates	0.40%		0.37%
Borrowings With Maturities Of 31 To 90 Days [Member] | Agency Securities [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	1,021,416		619,710
Accrued Interest Receivable	2,117		1,551
Borrowings Outstanding	968,370		594,283
Average Borrowing Rates	0.41%		0.32%
Collateral For Structured Financings [Member]
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral Carrying Amount	2,877		3,326
Borrowings Outstanding	$ 2,877		$ 3,326
Average Borrowing Rates	8.12%		8.04%

[1] After considering cash flow hedges that coincide with the floating rate terms of these borrowings that begin in 2015 and 2016, the effective borrowing rate during the final 20 years of these borrowings will average 7.56%, subject to certain adjustments for the effects of measured hedge ineffectiveness, if any.