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Summary of Significant Accounting Policies - Additional Information (Narrative) (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items] |  |
Subsidiary or Equity Method Investee, Cumulative Percentage Ownership after All Transactions | 83.68% |
Noncontrolling interest | $ 361,000 | $ 2,631,000
Cash equivalents maximum maturity period of highly liquid investments | 90 days |
Minimum [Member] |  |
Summary Of Significant Accounting Policies [Line Items] |  |
Number of business days for related transactions | 1 day |
Loan forgiven over service period | 3 years |
Minimum [Member] | Furniture and Fixtures [Member] |  |
Summary Of Significant Accounting Policies [Line Items] |  |
Depreciation and amortization on straight-line basis | 3 years |
Maximum [Member] |  |
Summary Of Significant Accounting Policies [Line Items] |  |
Number of business days for related transactions | 2 days |
Loan forgiven over service period | 5 years |
Maximum [Member] | Furniture and Fixtures [Member] |  |
Summary Of Significant Accounting Policies [Line Items] |  |
Depreciation and amortization on straight-line basis | 7 years |
Securities |  |
Summary Of Significant Accounting Policies [Line Items] |  |
Number of business days for related transactions | 2 days |
Commodities |  |
Summary Of Significant Accounting Policies [Line Items] |  |
Number of business days for related transactions | 1 day |

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