Fair Value of Financial Instruments (Details) (USD $)	Jun. 30, 2013		Dec. 31, 2012
Fair value and carrying values for notes and bonds payable, mortgage notes receivable and notes receivable
Notes and bonds payable, Carrying value	$ 1,301,387,000		$ 1,293,044,000
Mortgage notes receivable, Carrying value	212,313,000		162,191,000
Level 3
Fair value and carrying values for notes and bonds payable, mortgage notes receivable and notes receivable
Notes and bonds payable, Carrying value	1,301,400	[1]	1,293,000	[1]
Notes and bonds payable, Fair value	1,356,800	[1]	1,437,200	[1]
Level 2
Fair value and carrying values for notes and bonds payable, mortgage notes receivable and notes receivable
Mortgage notes receivable, Carrying value	212,300	[2]	162,200	[2]
Notes receivable, net of allowances, Carrying value	100	[2]	100	[2]
Mortgage notes receivable, Fair value	212,400	[2]	158,300	[2]
Notes receivable, net of allowances, Fair value	$ (100)	[2]	$ 100	[2]

[1] Level 3 - Fair value derived from valuation techniques in which one or more significant inputs or significant value drivers are unobservable.

[2] Level 2 - Fair value based on quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations in which significant inputs and significant value drivers are observable in active markets.