Securitized Loans Held for Investment - Summary (Detail) (Residential Mortgage Backed Securities [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Dec. 31, 2010
Residential Mortgage Backed Securities [Member]
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Securitized loans, at amortized cost	$ 1,148,187	$ 270,570
Less: allowance for loan losses	11,264	13,938	11,006
Securitized loans held for investment	$ 1,136,923	$ 256,632

The reserve to cover probable credit losses related to specifically identified fixed maturity real estate loans as well as probable credit losses inherent in the remainder of that loan portfolio.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 60

-Paragraph 47

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 310

-Section 45

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6873192&loc=d3e21338-158488

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