Fair Value of Financial Instruments (Details) (USD $)	Mar. 31, 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	$ 1,415,119,000	[1]	$ 1,293,044,000	[1]
Mortgage notes receivable	190,134,000	[2]	162,191,000	[2]
Notes receivable, net of allowances, Carrying value	100,000	[2]	100,000	[2]
Notes and bonds payable, Fair value	1,542,100,000	[1]	1,437,200,000	[1]
Mortgage notes receivable, Fair value	187,400,000	[2]	158,300,000	[2]
Notes receivable, net of allowances, Fair value	$ 100,000	[2]	$ 100,000	[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.