Mortgage-Backed Securities - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2012
Minimum
Investments Classified by Contractual Maturity Date [Line Items]
Contractual maturity of Company's agency securities, in years	15 years
Maximum
Investments Classified by Contractual Maturity Date [Line Items]
Contractual maturity of Company's agency securities, in years	30 years