Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Level 1
Assets:
U.S. Treasury securities		$ 752,076
Investment in affiliate	134,948	234,120
Other derivative contracts	940	7,955
Liabilities:
U.S. Treasury securities sold, not yet purchased		495,437
Level 2
Assets:
Agency mortgage-backed securities	92,487,318	123,963,207
Agency debentures	3,306,473	3,009,568
Interest rate swaps	38,950
Other derivative contracts	90,330	1,875
Liabilities:
Interest rate swaps	$ 1,189,178	$ 2,584,907