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Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2017
Summary of Significant Accounting Policies |
Summary of the impact of revisions in risk-free discount rate | The following table summarizes the impacts of revisions in the risk-free discount rate applied to our environmental remediation liabilities and recovery assets for the years ended December 31 (in millions) and the risk-free discount rate applied as of December 31:
|  | 2017 |  | 2016 |  | 2015 |
Decrease in operating expenses |  | $ | — |  | $ | (2) |  | $ | (2) |
Risk-free discount rate applied to environmental remediation liabilities and recovery assets |  |  | 2.5 | % |  | 2.5 | % |  | 2.25 | %
Schedule of estimated useful lives | The estimated useful lives for significant property and equipment categories are as follows (in years):
|  | Useful Lives
Vehicles — excluding rail haul cars |  | 3 to 10
Vehicles — rail haul cars |  | 10 to 20
Machinery and equipment — including containers |  | 3 to 30
Buildings and improvements |  | 5 to 40
Furniture, fixtures and office equipment |  | 3 to 10
Summary of Equity and Cost Method Investments | The following table summarizes our equity and cost method investments as of December 31 (in millions):
|  | 2017 |  | 2016
Equity method investments |  | $ | 127 |  | $ | 173
Cost method investments |  |  | 142 |  |  | 147
Investments in unconsolidated entities |  | $ | 269 |  | $ | 320
Schedule of Supplemental Cash Flow Information |
|  | 2017 |  | 2016 |  | 2015
Interest, net of capitalized interest |  | $ | 380 |  | $ | 375 |  | $ | 384
Income taxes |  |  | 562 |  |  | 442 |  |  | 419

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- Definition Impacts of revisions in the risk free discount rate applied to environmental remediation liabilities and recovery assets. + References No definition available. + Details
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