Notes Receivable Backed by Real Estate - Summary of Notes Receivable Backed by Real Estate (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended					9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 $188 million Preferred Equity Interest [Member]	Dec. 31, 2011 $188 million Preferred Equity Interest [Member]	Sep. 30, 2012 $55 million Preferred Equity Interest [Member]	Dec. 31, 2011 $55 million Preferred Equity Interest [Member]	Sep. 30, 2012 NAIF II Secured Mortgage Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 322,834	$ 188,009	$ 188,000	$ 55,970	$ 55,970	$ 78,864
Elimination upon acquisition of NAIF II	(78,864)					(78,864)
Accrued interest/(interest payments received), net	9	9
Ending balance	$ 243,979	$ 188,009	$ 188,000	$ 55,970	$ 55,970