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Summary of Significant Accounting Policies - Concentration Risk (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($) customer | Dec. 31, 2017 USD ($) customer
Concentrations of risk |  |
Accrued interest | $ | $ 2.3 | $ 1.7
Amounts on deposit in excess of federally insured limits approximately | $ | $ 26.0 | $ 16.3
Payables | Service providers or vendors |  |
Concentrations of risk |  |
Number of service providers | customer | 4 | 2
Concentration risk percentage | 49.00% | 40.00%
Number of critical suppliers | customer | 3 |

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