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Fair Value of Financial Instruments (Details) (USD $)
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12 Months Ended | |||||||
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Dec. 31, 2012
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Dec. 31, 2011
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Dec. 31, 2012
Carrying value per balance sheet
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Dec. 31, 2011
Carrying value per balance sheet
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Dec. 31, 2012
Estimated fair value
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Dec. 31, 2011
Estimated fair value
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Dec. 31, 2012
Estimated fair value
Low end of the range
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Dec. 31, 2012
Estimated fair value
Median of Range
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| Fair value of financial assets and liabilities | ||||||||
| Notes receivable issued in connection with property sales | $ 18,900,000 | $ 19,000,000 | $ 20,100,000 | $ 19,600,000 | ||||
| Notes receivable issued in connection with an acquisition | 8,780,000 | 8,780,000 | 8,800,000 | 8,800,000 | 8,800,000 | 8,800,000 | ||
| Mortgages payable assumed in connection with acquisitions | 175,900,000 | 67,800,000 | 176,700,000 | 68,200,000 | ||||
| Notes payable | $ 2,550,000,000 | $ 1,750,000,000 | $ 2,827,100,000 | $ 1,901,900,000 | ||||
| Length of Treasury yield curve used to calculate interest rate | 5 years | 7 years | ||||||