SIGNIFICANT ACCOUNTING POLICIES (Landfill Accounting) (Details) $ in Millions	12 Months Ended
	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