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SIGNIFICANT ACCOUNTING POLICIES (Tables) | 12 Months Ended
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Dec. 31, 2021
Accounting Policies [Abstract] |
Marketable Securities | As of December 31, 2021 and 2020, the Company had total marketable securities as follows (in thousands):
| December 31, 2021 |  | December 31, 2020
U.S. Treasury securities | $ | 901 |  |  | $ | 28,901 |
Municipal bonds | 1,978 |  |  | — |
Corporate notes and bonds | 57,685 |  |  | 22,956 |
Commercial paper | 21,160 |  |  | — |
Total marketable securities | $ | 81,724 |  |  | $ | 51,857 |
Schedule of Activity in Allowance for Doubtful Accounts | The following table reflects the activity in the allowance for doubtful accounts and revenue allowance (in thousands):
| Allowance for Doubtful Accounts |  | Revenue Allowance
| 2021 |  | 2020 |  | 2019 |  | 2021 |  | 2020 |  | 2019
Balance at January 1, | $ | 24,634 |  |  | $ | 22,493 |  |  | $ | 26,368 |  |  | $ | 20,115 |  |  | $ | 16,218 |  |  | $ | 17,947 |
Additions charged to earnings | 8,018 |  |  | 10,133 |  |  | 2,408 |  |  | 34,319 |  |  | 45,784 |  |  | 35,549 |
Deductions from reserves, net of recoveries | (8,516) |  |  | (7,992) |  |  | (6,283) |  |  | (38,430) |  |  | (41,887) |  |  | (37,278) |
Balance at December 31, | $ | 24,136 |  |  | $ | 24,634 |  |  | $ | 22,493 |  |  | $ | 16,004 |  |  | $ | 20,115 |  |  | $ | 16,218 |
Schedule of Activity in Revenue Allowance | The following table reflects the activity in the allowance for doubtful accounts and revenue allowance (in thousands):
| Allowance for Doubtful Accounts |  | Revenue Allowance
| 2021 |  | 2020 |  | 2019 |  | 2021 |  | 2020 |  | 2019
Balance at January 1, | $ | 24,634 |  |  | $ | 22,493 |  |  | $ | 26,368 |  |  | $ | 20,115 |  |  | $ | 16,218 |  |  | $ | 17,947 |
Additions charged to earnings | 8,018 |  |  | 10,133 |  |  | 2,408 |  |  | 34,319 |  |  | 45,784 |  |  | 35,549 |
Deductions from reserves, net of recoveries | (8,516) |  |  | (7,992) |  |  | (6,283) |  |  | (38,430) |  |  | (41,887) |  |  | (37,278) |
Balance at December 31, | $ | 24,136 |  |  | $ | 24,634 |  |  | $ | 22,493 |  |  | $ | 16,004 |  |  | $ | 20,115 |  |  | $ | 16,218 |
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, 2021 |  | December 31, 2020
Land | $ | 165,010 |  |  | $ | 139,776 |
Asset retirement costs (non-landfill) | 19,105 |  |  | 16,407 |
Landfill assets | 205,873 |  |  | 191,687 |
Buildings and improvements (1) | 551,795 |  |  | 509,804 |
Camp equipment | 127,680 |  |  | 159,021 |
Vehicles (2) | 912,836 |  |  | 844,026 |
Equipment (3) | 2,092,395 |  |  | 1,807,235 |
Furniture and fixtures | 6,444 |  |  | 7,082 |
Construction in progress | 60,447 |  |  | 24,378 |
| 4,141,585 |  |  | 3,699,416 |
Less - accumulated depreciation and amortization | 2,278,410 |  |  | 2,174,118 |
Total property, plant and equipment, net | $ | 1,863,175 |  |  | $ | 1,525,298 |
___________________________________ (1) Balances inclusive of gross ROU assets classified as finance leases of $8.9 million in both periods. (2) Balances inclusive of gross ROU assets classified as finance leases of $77.7 million and $47.2 million, respectively. (3) Balances inclusive of gross ROU assets classified as finance leases of $9.3 million both periods.
Schedule of Active Landfill Sites | As of December 31, 2021, 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 |  | 21 |  | 5,500 |
Deer Park |  | Texas |  | 1 |  | 32 |
Deer Trail |  | Colorado |  | 26 |  | 1,572 |
Grassy Mountain |  | Utah |  | 40 |  | 4,608 |
Kimball |  | Nebraska |  | 5 |  | 120 |
Lambton |  | Ontario, Canada |  | 48 |  | 4,534 |
Lone Mountain |  | Oklahoma |  | 16 |  | 3,617 |
Ryley |  | Alberta, Canada |  | 3 |  | 455 |
Sawyer |  | North Dakota |  | 84 |  | 3,346 |
|  |  |  |  |  | 23,784 |
Schedule of Remaining Highly Probable Airspace | The following table presents the remaining highly probable airspace from January 1, 2019 through December 31, 2021 (in thousands of cubic yards):
| 2021 |  | 2020 |  | 2019
Remaining capacity, beginning of year | 24,716 |  |  | 28,494 |  |  | 29,760 |
Changes in highly probable airspace, net | — |  |  | (2,962) |  |  | — |
Consumed | (932) |  |  | (816) |  |  | (1,266) |
Remaining capacity, end of year | 23,784 |  |  | 24,716 |  |  | 28,494 |

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