Estimated Fair Value for All Financial Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Financial assets:
U.S. Treasury securities	$ 1,117,915	$ 752,076
Agency mortgage-backed securities	70,388,949	123,963,207
Agency debentures	2,969,885	3,009,568
Investment in affiliates	139,447	234,120
Interest rate swaps	705,769	9,830
Other derivatives	146,725	9,830
Financial liabilities:
U.S. Treasury securities sold, not yet purchased	1,918,394	495,437
Securities loaned	2,527,668	1,808,315
Interest rate swaps	1,197,346	2,584,907
Other derivatives	55,518
Level 1
Financial assets:
U.S. Treasury securities	1,117,915	752,076
Investment in affiliates	139,447	234,120
Other derivatives	3,487	7,955
Financial liabilities:
U.S. Treasury securities sold, not yet purchased	1,918,394	495,437
Other derivatives	439
Level 2
Financial assets:
Agency mortgage-backed securities	70,388,949	123,963,207
Agency debentures	2,969,885	3,009,568
Other derivatives	143,238	1,875
Financial liabilities:
Other derivatives	55,079
Estimate of Fair Value, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents	552,436	615,789
Reverse repurchase agreements	100,000	1,811,095
Securities borrowed	2,582,893	2,160,942
U.S. Treasury securities	1,117,915	752,076
Investment in affiliates	139,447	234,120
Financial liabilities:
U.S. Treasury securities sold, not yet purchased	1,918,394	495,437
Securities loaned	2,527,668	1,808,315
Convertible Senior Notes	870,199	899,192
Estimate of Fair Value, Fair Value Disclosure | Level 2
Financial assets:
Agency mortgage-backed securities	70,388,949	123,963,207
Agency debentures	2,969,885	3,009,568
Corporate debt	118,362	64,271
Interest rate swaps	559,044
Financial liabilities:
Mortgages payable	19,240
Interest rate swaps	1,141,828	2,584,907
Estimate of Fair Value, Fair Value Disclosure | Level 3
Financial assets:
Commercial real estate debt and preferred equity	1,581,836
Financial liabilities:
Participation sold	14,050
Estimate of Fair Value, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives	146,725	9,830
Financial liabilities:
Repurchase agreements	62,134,133	103,332,832
Other derivatives	55,518
Carrying (Reported) Amount, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents	552,436	615,789
Reverse repurchase agreements	100,000	1,811,095
Securities borrowed	2,582,893	2,160,942
U.S. Treasury securities	1,117,915	752,076
Investment in affiliates	139,447	234,120
Financial liabilities:
U.S. Treasury securities sold, not yet purchased	1,918,394	495,437
Securities loaned	2,527,668	1,808,315
Convertible Senior Notes	825,262	825,541
Carrying (Reported) Amount, Fair Value Disclosure | Level 2
Financial assets:
Agency mortgage-backed securities	70,388,949	123,963,207
Agency debentures	2,969,885	3,009,568
Corporate debt	117,687	63,944
Interest rate swaps	559,044
Financial liabilities:
Mortgages payable	19,332
Interest rate swaps	1,141,828	2,584,907
Carrying (Reported) Amount, Fair Value Disclosure | Level 3
Financial assets:
Commercial real estate debt and preferred equity	1,583,969
Financial liabilities:
Participation sold	14,065
Carrying (Reported) Amount, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives	146,725	9,830
Financial liabilities:
Repurchase agreements	61,781,001	102,785,697
Other derivatives	$ 55,518