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Summary of Significant Accounting Policies - Landfill (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($) site | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Significant accounting policies |  |  |
Receivables related to income tax payments in excess of our provision for income taxes | $ 284 | $ 504 |
Environmental remediation reasonably possible additional losses high estimate | 140 |  |
Environmental remediation liabilities | 237 |  |
Environmental remediation liabilities not subject to inflation or discounting | 35 | 47 |
Increase of environmental remediation liability | $ 3 | $ 0 |
Landfill [Member] |  |  |
Significant accounting policies |  |  |
Required period to maintain and monitor landfill sites | 30 years |  |
Inflation rate | 2.50% | 2.50% | 2.50%
Credit adjusted, risk free discount rate applicable to long-term asset retirement obligations | 5.50% |  |
Number of landfills sites with expansion | site | 15 |  |
Number of expansion landfill sites require CFO approval inclusion of unpermitted airspace | site | 2 |  |
Decrease in operating expenses | $ (2) |  | $ (2)
Risk-free discount rate applied to environmental remediation liabilities and recovery assets | 2.75% | 2.50% | 2.50%
Landfill [Member] | Minimum [Member] |  |  |
Significant accounting policies |  |  |
Permit application process period requirement | 1 year |  |
Landfill [Member] | Maximum [Member] |  |  |
Significant accounting policies |  |  |
Permit application process period requirement | 5 years |  |
Accounts Receivable Net [Member] | Concentration of Credit Risk [Member] | Maximum [Member] |  |  |
Significant accounting policies |  |  |
Concentration of risk threshold (as a percent) | 5.00% | 5.00% |

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- Definition Number of landfill sites with expansions that require the Chief Financial Officer to approve the inclusion of the unpermitted airspace. + References No definition available. + Details
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