Agency Mortgage-Backed Securities - Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012 Securities	Dec. 31, 2011 Securities
Unrealized Loss Position For:
Unrealized loss position for less than 12 months - estimated fair value	$ 11,220,514	$ 1,087,552
Unrealized loss position for less than 12 months - unrealized losses	(82,721)	(118,593)
Unrealized loss position for less than 12 months -number of Securities	187	71
Unrealized loss Unrealized loss position for 12 months or more - estimated fair value	147,775	883,143
Unrealized loss position for 12 months or more - unrealized losses	(105,280)	(16,529)
Unrealized loss position for 12 months or more - number of Securities	39	36
Total unrealized loss position - estimated fair value	11,368,289	1,970,695
Total Unrealized Loss Position - Unrealized Losses	$ (188,001)	$ (135,122)
Total Unrealized Loss Position - number of Securities	226	107