Investments in Securities and Security-Related Receivables (Parenthetical) (Detail) (CMBS receivables, Security-related receivables, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Standard & Poor's AAA and A- Rating
Investment Securities [Line Items]
Securities, fair value	$ 8,176	$ 8,398
Standard & Poor's BBB+ and B- Rating
Investment Securities [Line Items]
Securities, fair value	25,992	26,013
Standard & Poor's, CCC Rating
Investment Securities [Line Items]
Securities, fair value	9,667	8,782
Standard & Poor's, D Rating
Investment Securities [Line Items]
Securities, fair value	$ 714	$ 617