Mortgage-Backed Securities - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified			6 Months Ended
Jun. 30, 2013 Other Forward Purchase Commitments	Dec. 31, 2012 Other Forward Purchase Commitments	Jun. 30, 2013 Minimum	Jun. 30, 2013 Maximum
Investments Classified by Contractual Maturity Date [Line Items]
Contractual maturity of Company's agency securities, in years			15 years	30 years
Derivatives	$ 13,458	$ 3,305

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Name:	us-gaap_DerivativeAssetsLiabilitiesAtFairValueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant