Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets:
U.S. Treasury securities		$ 1,117,915
Agency mortgage-backed securities	81,565,256	70,388,949
Agency debentures	1,368,350	2,969,885
Investment in affiliates	143,045	139,447
Interest rate swaps	75,225	559,044
Other derivatives	5,499	146,725
Total Assets	83,157,375	75,321,965
Liabilities:
U.S. Treasury securities sold, not yet purchased		1,918,394
Interest rate swaps	1,608,286	1,141,828
Other derivatives	8,027	55,518
Total Liabilities	1,616,313	3,115,740
Level 1
Assets:
U.S. Treasury securities		1,117,915
Investment in affiliates	143,045	139,447
Other derivatives	117	3,487
Total Assets	143,162	1,260,849
Liabilities:
U.S. Treasury securities sold, not yet purchased		1,918,394
Other derivatives	3,769	439
Total Liabilities	3,769	1,918,833
Level 2
Assets:
Agency mortgage-backed securities	81,565,256	70,388,949
Agency debentures	1,368,350	2,969,885
Interest rate swaps	75,225	559,044
Other derivatives	5,382	143,238
Total Assets	83,014,213	74,061,116
Liabilities:
Interest rate swaps	1,608,286	1,141,828
Other derivatives	4,258	55,079
Total Liabilities	$ 1,612,544	$ 1,196,907