Agency Mortgage-Backed Securities - Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013 Securities	Dec. 31, 2012 Securities
Unrealized Loss Position For:
Unrealized loss position for less than 12 months - estimated fair value	$ 39,235,229	$ 11,220,514
Unrealized loss position for less than 12 months - unrealized losses	(364,700)	(82,721)
Unrealized loss position for less than 12 months -number of Securities	376	187
Unrealized loss Unrealized loss position for 12 months or more - estimated fair value	120,653	147,775
Unrealized loss position for 12 months or more - unrealized losses	(74,573)	(105,280)
Unrealized loss position for 12 months or more - number of Securities	31	39
Total unrealized loss position - estimated fair value	39,355,882	11,368,289
Total Unrealized Loss Position - Unrealized Losses	$ (439,273)	$ (188,001)
Total Unrealized Loss Position - number of Securities	407	226