Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Assets:
U.S. Treasury securities		$ 1,117,915
Agency mortgage-backed securities	81,462,387	70,388,949
Agency debentures	1,334,181	2,969,885
Investment in affiliates	136,748	139,447
Interest rate swaps	198,066	559,044	[1]
Other derivatives	19,407	146,725
Total Assets	83,150,789	75,321,965
Liabilities:
U.S. Treasury securities sold, not yet purchased		1,918,394
Interest rate swaps	857,658	1,141,828	[1]
Other derivatives		55,518
Total Liabilities	857,658	3,115,740
Level 1
Assets:
U.S. Treasury securities		1,117,915
Investment in affiliates	136,748	139,447
Other derivatives	6,680	3,487
Total Assets	143,428	1,260,849
Liabilities:
U.S. Treasury securities sold, not yet purchased		1,918,394
Other derivatives		439
Total Liabilities		1,918,833
Level 2
Assets:
Agency mortgage-backed securities	81,462,387	70,388,949
Agency debentures	1,334,181	2,969,885
Interest rate swaps	198,066	559,044
Other derivatives	12,727	143,238
Total Assets	83,007,361	74,061,116
Liabilities:
Interest rate swaps	857,658	1,141,828
Other derivatives		55,079
Total Liabilities	$ 857,658	$ 1,196,907

[1] Derived from the audited consolidated financial statements at December 31, 2013.