Fair value measurements (Details) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Recurring
Marketable securities	$ 8,575,899	$ 8,570,338
Nonrecurring
Investment in affiliate	6,854,775	6,925,346
Fair Value, Inputs, Level 1 [Member]
Recurring
Marketable securities	8,575,899	8,570,338
Nonrecurring
Investment in affiliate	0	0
Fair Value, Inputs, Level 2 [Member]
Recurring
Marketable securities	0	0
Nonrecurring
Investment in affiliate	0	0
Fair Value, Inputs, Level 3 [Member]
Recurring
Marketable securities	0	0
Nonrecurring
Investment in affiliate	$ 6,854,775	$ 6,925,346

Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.4)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

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