Residential Mortgage Investments - Additional Information (Detail) (USD $) In Billions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013 M
Schedule Of Residential Mortgage Investments [Line Items]
Mortgage securities weighted average contractual maturity, months	292
Current-Reset ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Investments held, basis amount	$ 7.58
Agency securities average months to roll, months	5 months 21 days
Current-Reset ARMs [Member] | Maximum [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Agency securities months to roll	18 months
Longer-To-Reset ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Investments held, basis amount	$ 5.97
Agency securities average months to roll, months	42 months 6 days
Longer-To-Reset ARMs [Member] | Minimum [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Agency securities months to roll	18 months