SIGNIFICANT ACCOUNTING POLICIES (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Cash Equivalents and Marketable Securities

As of December 31, 2018 and 2017 the Company has recorded total marketable securities and cash equivalents of $61.0 million and $40.2 million, respectively, as shown below (in thousands).

(2) SIGNIFICANT ACCOUNTING POLICIES (Continued)




	December 31, 2018		December 31, 2017
Commercial paper	$	8,126	$	2,048
Total cash equivalents	8,126		2,048

U.S. Treasury securities	10,133		3,855
Corporate notes and bonds	38,036		32,729
Commercial paper	4,687		1,595
Total short-term marketable securities	52,856		38,179
Total financial assets	$	60,982	$	40,227

Schedule of Asset Classification and Estimated Useful Life

The Company depreciates and amortizes the 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, 2018		December 31, 2017
Land	$	123,734	$	121,658
Asset retirement costs (non-landfill)	15,148		14,593
Landfill assets	154,918		144,539
Buildings and improvements	440,188		414,384
Camp and lodging equipment	152,998		170,012
Vehicles	721,735		617,959
Equipment	1,697,490		1,644,102
Furniture and fixtures	5,453		5,708
Construction in progress	20,931		57,618
	3,332,595		3,190,573
Less - accumulated depreciation and amortization	1,770,617		1,603,208
Total property, plant and equipment, net	$	1,561,978	$	1,587,365

Schedule of Active Landfill Sites

As of December 31, 2018, the Company had 11 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:



		Remaining Lives (Years)	Remaining Highly Probable Airspace (cubic yards) (in thousands)
Facility Name	Location	Remaining Lives (Years)	Permitted	Unpermitted	Total
Altair	Texas	3	311	—	311
Buttonwillow	California	21	6,234	—	6,234
Deer Park	Texas	4	141	—	141
Deer Trail	Colorado	28	1,755	—	1,755
Grassy Mountain	Utah	51	194	4,830	5,024
Kimball	Nebraska	9	190	—	190
Lambton	Ontario	64	4,821	—	4,821
Lone Mountain	Oklahoma	18	4,124	—	4,124
Ryley	Alberta	7	906	—	906
Sawyer	North Dakota	87	3,522	—	3,522
Westmorland	California	64	2,732	—	2,732
			24,930	4,830	29,760

Remaining Highly Probable Airspace

The following table presents the remaining highly probable airspace from January 1, 2016 through December 31, 2018 (in thousands of cubic yards):



	2018		2017		2016
Remaining capacity at January 1,	31,113		32,228		29,786
Changes in highly probable airspace, net	(223	)	—		3,464
Consumed	(1,130	)	(1,115	)	(1,022	)
Remaining capacity at December 31,	29,760		31,113		32,228