Borrowings - Summary of Outstanding Balances, Maximum and Average Balances of Borrowings (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Balance	$ 211,944	$ 225,158
Wells Facility borrowings [Member]
Debt Instrument [Line Items]
Balance	211,941	225,155
Maximum Month-End Balance	225,155
Average Month-End Balance	216,947
JPMorgan Facility borrowings [Member]
Debt Instrument [Line Items]
Balance	3	3
Maximum Month-End Balance	3
Average Month-End Balance	$ 3

Carrying value, as of the balance sheet date, of securities sold under agreements to repurchase where the transferor maintains effective control over the assets, accounting for them as secured debt.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(i))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 10

-Section 55

-Paragraph 51

-URI http://asc.fasb.org/extlink&oid=7580763&loc=d3e101761-111710

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 140

-Paragraph 48, 100

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_SecuredDebtRepurchaseAgreements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant