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Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2017
Accounting Policies [Abstract] |
Summary of Estimated Useful Life of Fixed Assets | Depreciation for fixed assets is computed using the straight-line method over the following estimated useful lives:
|  | Useful Life
Office equipment |  | Three to ten years
Furniture and fixtures |  | Three to ten years
Computer equipment |  | Three to five years
Building |  | Fifteen to forty years
Software |  | Three to five years
Refined fuel plants |  | Ten years
Leasehold improvements |  | Shorter of the lease term or useful life of the asset

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