Agency Debentures (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Agency Debentures:
Agency debentures - unrealized gain	$ 2,360,608,000		$ 2,360,608,000		$ 1,704,203,000
Agency Debentures [Member]
Agency Debentures:
Agency debentures, value	703,100,000		703,100,000		1,100,000,000
Agency debentures - unrealized gain	141,000		141,000		9,700,000
Agency debentures - sold or called	125,800,000	1,200,000,000	947,600,000	1,400,000,000
Agency debentures - gain from sale	$ 1,400,000	$ 1,200,000	$ 7,700,000	$ 1,200,000