Residential Mortgage Investments (Narrative) (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 M	Sep. 30, 2011	Dec. 31, 2009
Schedule Of Residential Mortgage Investments [Line Items]
Mortgage securities weighted average contractual maturity, months			295
Gains on unit sales	$ 41,000		$ 48,000
Operating costs		74,000		747,000
Impairment charge				470,000
Amount of investment written off					39,200,000
Amount of securities collateralized	1,800,000		1,800,000
Longer-To-Reset ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Investments held, basis amount	5,870,000,000		5,870,000,000
Agency securities average months to roll, months			44.1
Longer-To-Reset ARMs [Member] | Maximum [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Agency securities months to roll			18
Current-Reset ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Investments held, basis amount	$ 8,070,000,000		$ 8,070,000,000
Agency securities average months to roll, months			5.4
Current-Reset ARMs [Member] | Minimum [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Agency securities months to roll			18