Fair Value of Financial Instruments (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Fair value and carrying values for notes and bonds payable, mortgage notes receivable and notes receivable
Notes and bonds payable, Carrying value	$ 1,395,600,000	$ 1,393,537,000
Mortgage notes receivable	118,059,000	97,381,000
Notes receivable, net of allowances, Carrying value	200,000	300,000
Notes and bonds payable, Fair value	1,550,200,000	1,534,300,000
Mortgage notes receivable, Fair value	117,500,000	95,500,000
Notes receivable, net of allowances, Fair value	$ 200,000	$ 300,000