Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets:
U.S. Treasury securities		$ 1,117,915
Agency mortgage-backed securities	75,350,388	70,388,949
Agency debentures	2,408,259	2,969,885
Investment in affiliates	137,647	139,447
Other derivatives	40,105	146,725
Interest rate swaps	340,890	559,044	[1]
Total Assets	78,277,289	75,321,965
Liabilities:
Interest rate swaps	1,272,616	1,141,828	[1]
Other derivatives	6,045	55,518
Total Liabilities	1,278,661	3,115,740
U.S. Treasury securities sold, not yet purchased		1,918,394
Interest Rate Swaps
Assets:
Interest rate swaps	340,890	559,044
Liabilities:
Interest rate swaps	1,272,616	1,141,828
Level 1
Assets:
U.S. Treasury securities		1,117,915
Investment in affiliates	137,647	139,447
Other derivatives		3,487
Total Assets	137,647	1,260,849
Liabilities:
Other derivatives		439
Total Liabilities		1,918,833
U.S. Treasury securities sold, not yet purchased		1,918,394
Level 2
Assets:
Agency mortgage-backed securities	75,350,388	70,388,949
Agency debentures	2,408,259	2,969,885
Other derivatives	40,105	143,238
Total Assets	78,139,642	74,061,116
Liabilities:
Other derivatives	6,045	55,079
Total Liabilities	1,278,661	1,196,907
Level 2 | Interest Rate Swaps
Assets:
Interest rate swaps	340,890	559,044
Liabilities:
Interest rate swaps	$ 1,272,616	$ 1,141,828

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