Estimated Fair Value for All Financial Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Financial liabilities:
Securities loaned	$ 1,248,968		$ 804,901	[1]
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Financial assets:
Cash and cash equivalents	2,264,854	[2]	994,198	[2]
Reverse repurchase agreements	1,612,384	[2]	860,866	[2]
U.S. Treasury Securities	2,242,039	[3]	928,547	[3]
Securities borrowed	1,602,692	[3]	928,732	[3]
Agency mortgage-backed securities	129,597,714		104,251,055
Agency debentures	2,935,538		889,580
Investment in affiliates	224,899	[3]	211,970	[3]
Receivable from Prime Broker	3,272		3,272
Equity securities			3,891	[3]
Corporate debt	64,928	[4]	52,073	[4]
Other derivatives	559	[3]	113	[3]
Financial liabilities:
U.S. Treasury Securities sold, not yet purchased	1,418,750	[3]	826,912	[3]
Repurchase agreements	101,033,146	[2],[5]	84,097,885	[2],[5]
Securities loaned	1,248,968	[3]	804,901	[3]
Convertible Senior Notes	999,749	[3]	539,913	[3]
Interest rate swaps	2,926,461		2,552,687
Estimate of Fair Value, Fair Value Disclosure [Member]
Financial assets:
Cash and cash equivalents	2,264,854	[2]	994,198	[2]
Reverse repurchase agreements	1,612,384	[2]	860,866	[2]
U.S. Treasury Securities	2,242,039	[3]	928,547	[3]
Securities borrowed	1,602,692	[3]	928,732	[3]
Agency mortgage-backed securities	129,597,714		104,251,055
Agency debentures	2,935,538		889,580
Investment in affiliates	224,899	[3]	211,970	[3]
Receivable from Prime Broker	3,272		3,272
Equity securities			3,891	[3]
Corporate debt	66,195	[4]	52,628	[4]
Other derivatives	559	[3]	113	[3]
Financial liabilities:
U.S. Treasury Securities sold, not yet purchased	1,418,750	[3]	826,912	[3]
Repurchase agreements	101,634,836	[2],[5]	84,369,817	[2],[5]
Securities loaned	1,248,968	[3]	804,901	[3]
Convertible Senior Notes	1,180,525	[3]	685,500	[3]
Interest rate swaps	$ 2,926,461		$ 2,552,687

[1] Derived from the audited consolidated financial statements at December 31, 2011.

[2] The fair value of repurchase agreements with maturities less than one year approximates carrying 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.