SIGNIFICANT ACCOUNTING POLICIES (Tables)

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Schedule of Cash Equivalents and Marketable Securities

As of December 31, 2020 and 2019, the Company had

total marketable securities and cash equivalents as follows (in thousands):

	December 31, 2020		December 31, 2019
Commercial paper	$	—	$	2,395
Total cash equivalents	—		2,395
U.S. Treasury securities	28,901		12,406
Corporate notes and bonds	22,956		26,678
Commercial paper	—		3,337
Total marketable securities	51,857		42,421
Total	$	51,857	$	44,816

Schedule of Asset Classification and Estimated Useful Life

The Company depreciates and amortizes the capitalized cost

of these assets, using the straight-line method as follows:

Asset Classification	Estimated Useful Life
Buildings and building improvements
Buildings	20-42 years
Leasehold and building improvements	2-45 years
Camp and lodging equipment	8-15 years
Vehicles	2-15 years
Equipment
Capitalized software and computer equipment	3-5 years
Containers and railcars	8-16 years
All other equipment	4-30 years
Furniture and fixtures	5-8 years

Property, plant and equipment consisted of the following (in thousands):

	December 31, 2020		December 31, 2019
Land	$	139,776	$	131,023
Asset retirement costs (non-landfill)	16,407		15,924
Landfill assets	191,687		182,276
Buildings and improvements (1)	509,804		499,159
Camp equipment	159,021		158,277
Vehicles (2)	844,026		785,056
Equipment (3)	1,807,235		1,779,366
Furniture and fixtures	7,082		6,054
Construction in progress	24,378		36,679
	3,699,416		3,593,814
Less - accumulated depreciation and amortization	2,174,118		2,005,663
Total property, plant and equipment, net	$	1,525,298	$	1,588,151

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(1) Balances inclusive of gross ROU assets classified as finance leases of $8.9 million and $31.0 million, respectively.

(2) Balances inclusive of gross ROU assets classified as finance leases of $47.2 million and $2.4 million, respectively.

(3) December 31, 2020 balance inclusive of gross ROU assets classified as finance leases of $9.3 million.

Schedule of Active Landfill Sites

As of December 31, 2020, the Company had 9 active landfill sites (including the Company's two non-commercial landfills), which have estimated remaining lives (based on anticipated waste volumes and remaining highly probable airspace) as follows:

Facility Name	Location	Remaining Lives (Years)	Permitted Remaining Highly Probable Airspace (cubic yards) (in thousands)
Buttonwillow	California	22	5,739
Deer Park	Texas	2	67
Deer Trail	Colorado	27	1,629
Grassy Mountain	Utah	41	4,710
Kimball	Nebraska	6	144
Lambton	Ontario, Canada	51	4,633
Lone Mountain	Oklahoma	32	3,803
Ryley	Alberta, Canada	5	575
Sawyer	North Dakota	68	3,416
			24,716

Schedule of Remaining Highly Probable Airspace

The following table presents the remaining highly probable airspace from January 1, 2018 through December 31, 2020 (in thousands of cubic yards):

	2020	2019	2018
Remaining capacity, beginning of year	28,494	29,760	31,113
Changes in highly probable airspace, net	(2,962)	—	(223)
Consumed	(816)	(1,266)	(1,130)
Remaining capacity, end of year	24,716	28,494	29,760