Investment Properties and Properties Held for Sale - Components of Properties Held for Sale (Detail) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Balance Sheet:
Land	$ 2,956,118	$ 2,956,118
Properties held for sale	0	33,991
Properties Held For Sale [Member]
Balance Sheet:
Land	0	33,991
Properties held for sale	$ 0	$ 33,991

Amount before accumulated depletion of real estate held for productive use, excluding land held for sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section 45

-Paragraph 4

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6812-107765

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.13)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_Land
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant