Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
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This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a recurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

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Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

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The aggregate value of all restricted investments.

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Financial Instruments, Risk Management and Fair Value measurements - Narrative (Details) mmbtus in Millions	Dec. 31, 2017 USD ($) mmbtus	Dec. 31, 2016 USD ($)	May 16, 2016 USD ($)
Financial Liabilities
Fair value of derivative (less than)	$ 100,000
Assets fair value	0
Significant transfers	0
Restricted investment	71,300,000	$ 69,700,000
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring
Financial Liabilities
Restricted investment	71,300,000	69,700,000
Restricted investments, fair value	69,600,000	67,100,000	$ 68,900,000
Reported Value
Financial Liabilities
Capital lease obligations	80,000,000	80,000,000
Estimate of Fair Value
Financial Liabilities
Capital lease obligations	$ 92,900,000	91,600,000
Commodity swap contracts
Financial Liabilities
Notional volume (in mmBTUS) \| mmbtus	1.4
Zero cost collar option contract
Financial Liabilities
Notional volume (in mmBTUS) \| mmbtus	1.4
Term Loan Facility
Financial Liabilities
Carrying amount of long-term debt	$ 365,600,000	$ 404,400,000