Estimated Fair Value for All Financial Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Financial liabilities:
Securities loaned	$ 1,808,315		$ 804,901
Level 1 [Member]
Financial assets:
U.S. Treasury Securities	752,076		928,547
Investment in affiliates	234,120		211,970
Equity securities			3,891
Other derivatives	7,955		113
Financial liabilities:
U.S. Treasury Securities sold, not yet purchased	495,437		826,912
Level 2 [Member]
Financial assets:
Agency mortgage-backed securities	123,963,207		104,251,055
Agency debentures	3,009,568		889,580
Other derivatives	1,875
Financial liabilities:
Interest rate swaps	2,584,907		2,552,687
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Financial assets:
Cash and cash equivalents			994,198	[1]
Reverse repurchase agreements			860,866	[1]
U.S. Treasury Securities			928,547	[2]
Securities borrowed			928,732	[2]
Agency mortgage-backed securities			104,251,055
Agency debentures			889,580
Investment in affiliates			211,970	[2]
Equity securities			3,891	[2]
Corporate debt			52,073	[3]
Other derivatives			113	[2]
Financial liabilities:
U.S. Treasury Securities sold, not yet purchased			826,912	[2]
Repurchase agreements			84,097,885	[1],[4]
Securities loaned			804,901	[2]
Convertible Senior Notes			539,913	[2]
Interest rate swaps			2,552,687
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Level 1 [Member]
Financial assets:
Cash and cash equivalents	615,789	[1]
Reverse repurchase agreements	1,811,095	[1]
U.S. Treasury Securities	752,076	[2]
Investment in affiliates	234,120	[2]
Financial liabilities:
U.S. Treasury Securities sold, not yet purchased	495,437	[2]
Convertible Senior Notes	825,541	[2]
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Level 2 [Member]
Financial assets:
Securities borrowed	2,160,942	[2]
Agency mortgage-backed securities	123,963,207
Agency debentures	3,009,568
Corporate debt	63,944	[3]
Financial liabilities:
Securities loaned	1,808,315	[2]
Interest rate swaps	2,584,907
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Level 1, Level 2 [Member]
Financial assets:
Other derivatives	9,830	[2]
Financial liabilities:
Repurchase agreements	102,785,697	[1],[4]
Estimate of Fair Value, Fair Value Disclosure [Member]
Financial assets:
Cash and cash equivalents			994,198	[1]
Reverse repurchase agreements			860,866	[1]
U.S. Treasury Securities			928,547	[2]
Securities borrowed			928,732	[2]
Agency mortgage-backed securities			104,251,055
Agency debentures			889,580
Investment in affiliates			211,970	[2]
Equity securities			3,891	[2]
Corporate debt			52,628	[3]
Other derivatives			113	[2]
Financial liabilities:
U.S. Treasury Securities sold, not yet purchased			826,912	[2]
Repurchase agreements			84,369,817	[1],[4]
Securities loaned			804,901	[2]
Convertible Senior Notes			685,500	[2]
Interest rate swaps			2,552,687
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 1 [Member]
Financial assets:
Cash and cash equivalents	615,789	[1]
Reverse repurchase agreements	1,811,095	[1]
U.S. Treasury Securities	752,076	[2]
Investment in affiliates	234,120	[2]
Financial liabilities:
U.S. Treasury Securities sold, not yet purchased	495,437	[2]
Convertible Senior Notes	899,192	[2]
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 2 [Member]
Financial assets:
Securities borrowed	2,160,942	[2]
Agency mortgage-backed securities	123,963,207
Agency debentures	3,009,568
Corporate debt	64,271	[3]
Financial liabilities:
Securities loaned	1,808,315	[2]
Interest rate swaps	2,584,907
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 1, Level 2 [Member]
Financial assets:
Other derivatives	9,830	[2]
Financial liabilities:
Repurchase agreements	$ 103,332,832	[1],[4]

[1] The fair value of cash accounts and repurchase agreements with maturities less than one year approximates carrying value due to the short-term maturities of these items.

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

[3] 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.

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