Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
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
Other derivatives	146,725	9,830
Interest rate swaps	559,044
Total Assets	75,321,965	127,968,801
Liabilities:
U.S. Treasury securities sold, not yet purchased	1,918,394	495,437
Interest rate swaps	1,141,828	2,584,907
Other derivatives	55,518
Total Liabilities	3,115,740	3,080,344
Interest Rate Swaps
Assets:
Interest rate swaps	559,044
Liabilities:
Interest rate swaps	1,141,828	2,584,907
Level 1
Assets:
U.S. Treasury securities	1,117,915	752,076
Investment in affiliates	139,447	234,120
Other derivatives	3,487	7,955
Total Assets	1,260,849	994,151
Liabilities:
U.S. Treasury securities sold, not yet purchased	1,918,394	495,437
Other derivatives	439
Total Liabilities	1,918,833	495,437
Level 2
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
Total Assets	74,061,116	126,974,650
Liabilities:
Other derivatives	55,079
Total Liabilities	1,196,907	2,584,907
Level 2 | Interest Rate Swaps
Assets:
Interest rate swaps	559,044
Liabilities:
Interest rate swaps	$ 1,141,828	$ 2,584,907