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Commitments, Contingencies and Legal Proceedings - Additional Information (Detail) | 9 Months Ended | 12 Months Ended |  |  |
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Sep. 30, 2018 USD ($) Operable_Unit | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Jul. 13, 2018 USD ($) | Jun. 30, 2018 USD ($) | Dec. 31, 2017 USD ($)
Commitments Contingencies And Litigation [Line Items] |  |  |  |  |  |
Number of operable units | Operable_Unit | 5 |  |  |  |  |
Term of agreement wih third party for monitoring and maintenance in OU1-OU4a | 30 years |  |  |  |  |
Assumption of escrow | $ 4,746,000 |  |  |  |  |
Reserve for Environmental liabilities, current portion | 26,000,000 |  |  |  |  | $ 28,500,000
Other Long-Term Liabilities [Member] |  |  |  |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |  |  |  |
Reserve for Environmental liabilities, remaining portion | 19,000,000 |  |  |  |  |
NCR/Appvion Consent Decree [Member] |  |  |  |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |  |  |  |
Contribution payment towards remedial action and natural resource damages | 75,000,000 |  |  |  |  |
Anticipated cost to complete work for remaining site remedy | 200,000,000 |  |  |  |  |
Expected payments for governments unreimbursed past costs which are under dispute | 38,500,000 |  |  |  |  |
Unreimbursed recoverable cost |  |  |  |  | $ 0 |
Unspent settlement money |  |  |  |  | $ 500,000 |
NADAR [Member] |  |  |  |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |  |  |  |
Expected payments for governments unreimbursed past costs which are under dispute | 105,000,000 |  |  | $ 105,000,000 |  |
Unspent settlement money | 60,000,000 |  |  |  |  |
Georgia Pacific [Member] |  |  |  |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |  |  |  |
Settlement payable amount | $ 9,500,000 |  |  |  |  |
Payment due date | 2017-08 |  |  |  |  |
Maximum [Member] |  |  |  |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |  |  |  |
NCR costs and damages percentage | 100.00% |  |  |  |  |
Maximum [Member] | NCR/Appvion Consent Decree [Member] |  |  |  |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |  |  |  |
Estimated cost related to Fox River matter | $ 30,000,000 |  |  |  |  |
Minimum [Member] | December 2009 - Long Term Monitoring Plan [Member] |  |  |  |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |  |  |  |
Period over which cash expenditure to be incurred for monitoring activities | 30 years |  |  |  |  |
Minimum [Member] | GLT, GP and NCR [Member] | NCR/Appvion Consent Decree [Member] |  |  |  |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |  |  |  |
Contribution payment towards remedial action and natural resource damages | $ 1,000,000,000 |  |  |  |  |
OU2-5 [Member] |  |  |  |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |  |  |  |
Estimated future cost of cleanup work |  | $ 13,400,000 | $ 13,400,000 |  |  |
Expected cleanup work continuation year | through 2019 |  |  |  |  |
NCR costs and damages percentage | 100.00% |  |  |  |  |
OU2-5 [Member] | Whiting Litigation [Member] |  |  |  |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |  |  |  |
NCR costs and damages percentage | 100.00% |  |  |  |  |

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