Estimated Fair Value for All Financial Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Financial liabilities:
Securities loaned	$ 2,330,060		$ 1,808,315	[1]
Level 1
Financial assets:
U.S. Treasury Securities	1,645,930		752,076
Investment in affiliates	267,547		234,120
Other derivatives			7,955
Financial liabilities:
U.S. Treasury Securities sold, not yet purchased	611,167		495,437
Other derivatives	4,812
Level 2
Financial assets:
Agency mortgage-backed securities	108,256,671		123,963,207
Agency debentures	3,970,279		3,009,568
Other derivatives			1,875
Financial liabilities:
Interest rate swaps	2,259,173		2,584,907
Carrying (Reported) Amount, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents	1,862,550	[2]	615,789	[2]
Reverse repurchase agreements	4,933,465	[2]	1,811,095	[2]
U.S. Treasury Securities	1,645,930	[3]	752,076	[3]
Investment in affiliates	267,547	[3]	234,120	[3]
Financial liabilities:
U.S. Treasury Securities sold, not yet purchased	611,167	[3]	495,437	[3]
Convertible Senior Notes	824,902	[3]	825,541	[3]
Carrying (Reported) Amount, Fair Value Disclosure | Level 2
Financial assets:
Securities borrowed	2,688,485	[3]	2,160,942	[3]
Agency mortgage-backed securities	108,256,671		123,963,207
Agency debentures	3,970,279		3,009,568
Corporate debt	66,539	[4]	63,944	[4]
Financial liabilities:
Securities loaned	2,330,060	[3]	1,808,315	[3]
Interest rate swaps	2,259,173		2,584,907
Carrying (Reported) Amount, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives			9,830	[3]
Financial liabilities:
Repurchase agreements	100,322,942	[2],[5]	102,785,697	[2],[5]
Other derivatives	4,812	[3]
Estimate of Fair Value, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents	1,862,550	[2]	615,789	[2]
Reverse repurchase agreements	4,933,465	[2]	1,811,095	[2]
U.S. Treasury Securities	1,645,930	[3]	752,076	[3]
Investment in affiliates	267,547	[3]	234,120	[3]
Financial liabilities:
U.S. Treasury Securities sold, not yet purchased	611,167	[3]	495,437	[3]
Convertible Senior Notes	909,255	[3]	899,192	[3]
Estimate of Fair Value, Fair Value Disclosure | Level 2
Financial assets:
Securities borrowed	2,688,485	[3]	2,160,942	[3]
Agency mortgage-backed securities	108,256,671		123,963,207
Agency debentures	3,970,279		3,009,568
Corporate debt	67,130	[4]	64,271	[4]
Financial liabilities:
Securities loaned	2,330,060	[3]	1,808,315	[3]
Interest rate swaps	2,259,173		2,584,907
Estimate of Fair Value, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives			9,830	[3]
Financial liabilities:
Repurchase agreements	100,828,852	[2],[5]	103,332,832	[2],[5]
Other derivatives	$ 4,812	[3]

[1] Derived from the audited consolidated financial statements at December 31, 2012.

[2] Carrying value approximates fair value due to the short-term maturities of these items.

[3] Fair value is determined using end of day quoted prices in active markets.

[4] The carrying value of the corporate debt is based on amortized cost. Estimates of fair value of corporate debt require the use of significant judgments and inputs including, but not limited to, the enterprise value of the borrower (i.e., an estimate of the total fair value of the borrower's debt and equity), the nature and realizable value of any collateral, the borrower's ability to make payments when due and its earnings history. Management also considers factors that affect the macro and local economic markets in which the borrower operates.

[5] The fair value of repurchase agreements with maturities greater than one year are valued as pay fixed versus receive floating interest rate swaps.