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SIGNIFICANT ACCOUNTING POLICIES (Tables) | 12 Months Ended
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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 |
___________________________________ (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 |

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