Investments in Tax-Exempt Bonds Avistar Portfolio (Details) (USD $)	3 Months Ended
Mar. 31, 2013
Avistar on the Boulevard [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of Units in Real Estate Property	344
Avistar at Chase Hill [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of Units in Real Estate Property	232
Avistar at the Crest [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of Units in Real Estate Property	200
Series A [Member] \| Avistar on the Boulevard [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	13,800,000
Series A [Member] \| Avistar at Chase Hill [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	9,000,000
Series A [Member] \| Avistar at the Crest [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	8,800,000
Series A [Member] \| Avistar Portfolio [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.00%
Available-for-sale Security, Maturity Date at a point in time	Mar. 01, 2050
Series B [Member] \| Avistar on the Boulevard [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	3,200,000
Series B [Member] \| Avistar at Chase Hill [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	2,000,000
Series B [Member] \| Avistar at the Crest [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	1,700,000
Series B [Member] \| Avistar Portfolio [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	9.00%
Available-for-sale Security, Maturity Date at a point in time	Apr. 01, 2050
Series C [Member] \| Avistar Portfolio [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	9.00%
Available-for-sale Security, Maturity Date at a point in time	Apr. 01, 2050
Available-for-sale Securities, at Par Value	804,000

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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+ Details