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Summary of Significant Accounting Policies - Landfill (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($) item site | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Significant accounting policies |  |  |
Number of expansion landfill sites require CFO approval inclusion of unpermitted airspace | item | 2 |  |
Environmental remediation reasonably possible additional losses high estimate | $ 140 |  |
Environmental remediation liabilities | 230 |  |
Environmental remediation liabilities not subject to inflation or discounting | 34 | $ 36 |
Increase (decrease) in environmental remediation liabilities | $ (12) | $ (8) |
Landfill [Member] |  |  |
Significant accounting policies |  |  |
Inflation rate | 2.25% | 2.50% | 2.50%
Credit adjusted, risk free discount rate applicable to long-term asset retirement obligations | 5.00% |  |
Number of landfills sites with expansion | site | 15 |  |
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% |
Environmental Remediation Liabilities [Member] |  |  |
Significant accounting policies |  |  |
Environmental remediation liabilities | $ 230 | $ 240 |
Increase (decrease) in operating expenses | $ 8 | $ 9 | $ (2)
Risk-free discount rate applied to environmental remediation liabilities and recovery assets | 1.00% | 1.75% | 2.75%

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