Mortgage-Backed Securities - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2012
Minimum
Investments Classified by Contractual Maturity Date [Line Items]
Lower range of contractual maturity of Company's agency MBS, in years	15 years
Maximum
Investments Classified by Contractual Maturity Date [Line Items]
Lower range of contractual maturity of Company's agency MBS, in years	30 years