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Summary of Significant Accounting Policies - Concentration of credit risk (Details) (USD $)
In Millions, unless otherwise specified |
6 Months Ended |
|---|---|
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Jun. 30, 2013
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| Concentration Risk [Line Items] | |
| Cash on deposit, number of financial institutions which had deposits in excess of current federally insured limits (in financial institutions) | 3 |
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Credit concentration risk | Cash on deposit
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|
| Concentration Risk [Line Items] | |
| Cash on deposit in excess of federally insured levels | $ 21.0 |