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Fair value measurements - Fair Value Measurements of Assets Measured at Fair Value on a Recurring Basis (Details) (Fair Value, Measurements, Recurring, USD $)
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Dec. 31, 2012
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Dec. 31, 2011
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Dec. 31, 2010
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Fair Value Total
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | |||
| Certificates of deposit | $ 181,859 | $ 683,428 | $ 85,000 |
| Total Assets | 181,859 | 683,428 | 85,000 |
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Quoted Prices in Active Markets (Level 1)
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | |||
| Certificates of deposit | 0 | 0 | 0 |
| Total Assets | 0 | 0 | 0 |
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Significant Other Observable Inputs (Level 2)
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | |||
| Certificates of deposit | 181,859 | 683,428 | 85,000 |
| Total Assets | 181,859 | 683,428 | 85,000 |
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Significant Unobservable Inputs (Level 3)
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | |||
| Certificates of deposit | 0 | 0 | 0 |
| Total Assets | $ 0 | $ 0 | $ 0 |