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SIGNIFICANT ACCOUNTING POLICIES (Tables) | 12 Months Ended
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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 |

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- Definition Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62557-112803 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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