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SIGNIFICANT ACCOUNTING POLICIES (Landfill Accounting) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($) cubic_yard cubicYard landfill_site | Dec. 31, 2019 USD ($) cubicYard cubic_yard | Dec. 31, 2018 USD ($) cubicYard cubic_yard
Property, Plant and Equipment [Line Items] |  |  |
Landfill sites | landfill_site | 9 |  |
Permitted, but not highly probable airspace (cubic yards) | 0 | 0 |
Remaining Airspace Capacity [Roll Forward] |  |  |
Remaining capacity, beginning of period (cubic yards) | 28,494,000 | 29,760,000 | 31,113,000
Changes in highly probable airspace, net (cubic yards) | (2,962,000) | 0 | (223,000)
Consumed (cubic yards) | (816,000) | (1,266,000) | (1,130,000)
Remaining capacity, end of period (cubic yards) | 24,716,000 | 28,494,000 | 29,760,000
Number of landfill sites closed during period | landfill_site | 2 |  |
Landfill final closure and post-closure liabilities | $ | $ 48.4 | $ 39.4 |
Regulatory post-closure period for landfill (in years) | 30 years |  |
Asset retirement obligations, inflation rate (as a percent) | 1.02% | 1.02% |
Asset retirement obligations, risk free rate (as a percent) | 5.60% | 6.02% |
Non-landfill closure and post-closure liabilities | $ | $ 39.5 | $ 36.3 |
Minimum |  |  |
Remaining Airspace Capacity [Roll Forward] |  |  |
Non-landfill closure and post-closure liabilities, period (in years) | 10 years |  |
Maximum |  |  |
Remaining Airspace Capacity [Roll Forward] |  |  |
Non-landfill closure and post-closure liabilities, period (in years) | 30 years |  |
Buttonwillow | California |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining Lives (Years) | 22 years |  |
Deer Park | Texas |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining Lives (Years) | 2 years |  |
Deer Trail | Colorado |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining Lives (Years) | 27 years |  |
Grassy Mountain | Utah |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining Lives (Years) | 41 years |  |
Kimball | Nebraska |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining Lives (Years) | 6 years |  |
Lambton | Ontario, Canada |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining Lives (Years) | 51 years |  |
Lone Mountain | Oklahoma |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining Lives (Years) | 32 years |  |
Ryley | Alberta, Canada |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining Lives (Years) | 5 years |  |
Sawyer | North Dakota |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining Lives (Years) | 68 years |  |
Altair |  |  |
Remaining Airspace Capacity [Roll Forward] |  |  |
Airspace consumed (in cubic yards) | cubicYard | 25,000 | 132,000 | 127,000
Westmorland |  |  |
Remaining Airspace Capacity [Roll Forward] |  |  |
Airspace consumed (in cubic yards) | cubicYard | 0 | 0 | 0
Landfill assets |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Amortization | $ | $ 10.9 | $ 12.3 | $ 10.3
Non-commercial landfills |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Landfill sites | landfill_site | 2 |  |
Permitted |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining highly probable airspace | 24,716,000 |  |
Permitted | Buttonwillow | California |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining highly probable airspace | 5,739,000 |  |
Permitted | Deer Park | Texas |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining highly probable airspace | 67,000 |  |
Permitted | Deer Trail | Colorado |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining highly probable airspace | 1,629,000 |  |
Permitted | Grassy Mountain | Utah |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining highly probable airspace | 4,710,000 |  |
Permitted | Kimball | Nebraska |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining highly probable airspace | 144,000 |  |
Permitted | Lambton | Ontario, Canada |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining highly probable airspace | 4,633,000 |  |
Permitted | Lone Mountain | Oklahoma |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining highly probable airspace | 3,803,000 |  |
Permitted | Ryley | Alberta, Canada |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining highly probable airspace | 575,000 |  |
Permitted | Sawyer | North Dakota |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining highly probable airspace | 3,416,000 |  |

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