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Summary of Significant Accounting Policies - Concentration of credit risk (Details) (USD $)
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3 Months Ended | 6 Months Ended | 0 Months Ended | ||
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Jun. 30, 2012
Property
financial_institutions
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Jun. 30, 2011
Property
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Jun. 30, 2012
Property
financial_institutions
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Jun. 30, 2011
Property
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Jun. 29, 2012
Credit concentration risk
Cash on deposit
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| Concentration Risk [Line Items] | |||||
| Cash on deposit, number of financial institutions | 4 | 4 | |||
| Cash on deposit, number of financial institutions which had deposits in excess of current federally insured limits (in financial institutions) | 4 | 4 | |||
| Cash on deposit in excess of federally insured levels | $ 14,800,000 | ||||
| Number of properties impaired | 1 | 0 | 1 | 0 | |
| Impairment of Real Estate | $ 1,979,000 | $ 0 | $ 1,979,000 | $ 0 | |