Fair Value of Financial Instruments and Investments - Summary of Notes Payable and Estimated Fair Value (Detail) (USD $)	Sep. 30, 2012		Dec. 31, 2011
Notes Payable	$ 619,275,000		$ 630,146,000
Note Payable at Fair Value	9,208,000		8,775,000
Book Value [Member]
Notes Payable	619,275,000	[1]	630,146,000	[1]
Note Payable at Fair Value	9,208,000	[1]	8,775,000	[1]
Total Notes Payable	628,483,000	[1]	638,921,000	[1]
Loan Payable	0	[1]	25,000,000	[1]
Fair Value [Member]
Notes Payable	654,145,000	[1]	656,135,000	[1]
Note Payable at Fair Value	9,208,000	[1]	8,775,000	[1]
Total Notes Payable	663,353,000	[1]	664,910,000	[1]
Loan Payable	$ 0	[1]	$ 25,000,000	[1]

[1] Level 3: For purposes of this fair value disclosure, we based our fair value estimate for notes payable on our internal valuation that includes a representative sample of our lenders' market interest rate quotes as of September 30, 2012 and December 31, 2011 for debt with similar risk characteristics and maturities. We based the Note Payable carried at fair value on a third party appraiser's valuation, which used similar techniques as our internal valuation model as of September 30, 2012 and December 31, 2011.