Residential Mortgage Investments (Narrative) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 M	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule Of Residential Mortgage Investments [Line Items]
Mortgage securities weighted average contractual maturity, months	293
Gains on unit sales	$ 44,000
Operating costs, net of gains on unit sales		277,000	436,000
Impairment charge		470,000	106,000
Amount of investment written off				39,200,000
Current-Reset ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Investments held, basis amount	7,820,000,000
Agency securities average months to roll, months	5.5
Current-Reset ARMs [Member] | Maximum [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Agency securities months to roll	18
Longer-To-Reset ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Investments held, basis amount	$ 5,700,000,000
Agency securities average months to roll, months	42.8
Longer-To-Reset ARMs [Member] | Minimum [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Agency securities months to roll	18