SIGNIFICANT ACCOUNTING POLICIES (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Available-for-sale Securities

Available-for-sale investments were as follows (in thousands):

	December 31, 2017
	Amortized Cost		Gross Unrealized Loss			Gross Unrealized Gain		Fair Value
Commercial paper	$	2,048	$	—		$	—	$	2,048
Total cash equivalents	2,048		—			—		2,048
U.S. Treasury securities	3,865		(10		)	—		3,855
Corporate notes and bonds	32,790		(61		)	—		32,729
Commercial paper	1,595		—			—		1,595
Total short-term marketable securities	38,250		(71		)	—		38,179
Total financial assets	$	40,298	$	(71	)	$	—	$	40,227

Marketable Securities

Marketable securities by contractual maturity as of December 31, 2017 are shown below (in thousands).

	Amortized Cost		Fair Value
Due in one year or less	$	18,684	$	18,670
After one year through three years	21,614		21,557
Total financial assets	$	40,298	$	40,227

Fair Value, Assets Measured on Recurring Basis

Financial assets measured and recorded at fair value on a recurring basis consisted of the following types of instruments (in thousands):

	December 31, 2017
	Level 1		Level 2		Level 3		Total
Commercial paper	$	—	$	2,048	$	—	$	2,048
Total cash equivalents	—		2,048		—		2,048
U.S. Treasury securities	3,855		—		—		3,855
Corporate notes and bonds	—		32,729		—		32,729
Commercial paper	—		1,595		—		1,595
Total short-term marketable securities	3,855		34,324		—		38,179
Total financial assets	$	3,855	$	36,372	$	—	$	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	30–42 years
Leasehold and building improvements	2–45 years
Camp equipment	8–15 years
Vehicles	3–15 years
Equipment
Capitalized software and computer equipment	3–5 years
Solar equipment	30 years
Containers and railcars	15–20 years
All other equipment	8–25 years
Furniture and fixtures	5–8 years

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

	December 31, 2017		December 31, 2016
Land	$	121,658	$	120,575
Asset retirement costs (non-landfill)	14,593		14,567
Landfill assets	144,539		139,708
Buildings and improvements	414,384		373,160
Camp equipment	170,012		152,740
Vehicles	617,959		541,022
Equipment	1,644,102		1,483,736
Furniture and fixtures	5,708		5,492
Construction in progress	57,618		146,904
	3,190,573		2,977,904
Less - accumulated depreciation and amortization	1,603,208		1,366,077
Total property, plant and equipment, net	$	1,587,365	$	1,611,827

Schedule of Active Landfill Sites

As of December 31, 2017, 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		Permitted	Unpermitted	Total
Altair	Texas	3	438	—	438
Buttonwillow	California	20	6,459	—	6,459
Deer Park	Texas	5	198	—	198
Deer Trail	Colorado	33	1,812	—	1,812
Grassy Mountain	Utah	59	291	4,830	5,121
Kimball	Nebraska	8	211	—	211
Lambton	Ontario	53	4,882	—	4,882
Lone Mountain	Oklahoma	24	4,402	—	4,402
Ryley	Alberta	8	351	880	1,231
Sawyer	North Dakota	84	3,627	—	3,627
Westmorland	California	64	2,732	—	2,732
			25,403	5,710	31,113

Remaining Highly Probable Airspace

The following table presents the remaining highly probable airspace from January 1, 2015 through December 31, 2017 (in thousands of cubic yards):

	2017		2016		2015
Remaining capacity at January 1,	32,228		29,786		30,544
Addition of highly probable airspace, net	—		3,464		516
Consumed	(1,115	)	(1,022	)	(1,274	)
Remaining capacity at December 31,	31,113		32,228		29,786