Residential Mortgage Investments (Narrative) (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012 Store	Jun. 30, 2011	Jun. 30, 2012 Month Store	Jun. 30, 2011	Dec. 31, 2009
Schedule Of Residential Mortgage Investments [Line Items]
Mortgage securities weighted average contractual maturity, months			293
Number of unsold completed units	2		2
Amount of securities collateralized	$ 529,000		$ 529,000
Gains on unit sales	108,000		7,000
Operating costs		570,000		673,000
Impairment charge				470,000
Amount of investment written off					39,200,000
Longer-To-Reset ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Investments held, basis amount	5,080,000,000		5,080,000,000
Agency securities average months to roll, months			45.0
Longer-To-Reset ARMs [Member] | Maximum
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	$ 8,410,000,000		$ 8,410,000,000
Agency securities average months to roll, months			5.4
Current-Reset ARMs [Member] | Minimum
Schedule Of Residential Mortgage Investments [Line Items]
Agency securities months to roll			18