Principles of consolidation and basis of presentation (Policies)	3 Months Ended
Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Environmental incidents	Environmental incidents: The Company accrues a liability when an environmental incident has occurred and the costs are estimable. The Company does not record a receivable for recoveries from third parties for environmental matters until it has determined that the amount of the collection is reasonably assured. The accrued liability is relieved when the Company pays the liability or a third party assumes the liability. Upon determination that collection is reasonably assured or a third party assumes the liability, the Company records the amount as a reduction of expense.
Recent accounting pronouncements	Recent accounting pronouncements: In February 2016, the FASB established Topic 842, Leases, by issuing Accounting Standards Update No. 2016-02, which requires lessors to classify leases as sales-type, direct financing, or operating leases. Topic 842 was subsequently amended by ASU No. 2018-01, Land Easement Practical Expedient for Transition to Topic 842; ASU No. 2018-10, Codification Improvements to Topic 842, Leases; and ASU No, 2018-111, Targeted Improvements. A lease is a sales-type lease if any one of five criteria are met, each of which indicate that the lease, in effect, transfers control of the underlying asset to the lessee. If none of the five criteria are met, but two additional criteria are both met, indicating that the lessor has transferred substantially all of the risks and benefits of the underlying asset to the lessee and a third party, the lease is a direct financing lease. All leases that are not sales-type or direct financing leases are operating leases. The new standard was effective for the Company on January 1, 2019. A modified retrospective transition approach was required by applying the new standard to all leases existing at the date of initial application. An entity may choose to use either (1) its effective date or (2) the beginning of the earliest comparative period presented as its date of initial application. The Company adopted the new standard on January 1, 2019 and used the effective date as our date of initial application. Consequently, financial information will not be updated and the disclosures required under the new standard will not be provided for dates and periods before January 1, 2019. The new standard provides a number of practical expedients in transition. The Company elected the “package of practical expedients”, which permits the Company not to reassess under the new standard prior conclusions about lease identification, lease classification and initial direct costs. While most of our leases will continue to be classified as operating leases in accordance with the package of practical expedients, leases that commence on or after the effective date of the new standard, and that would be classified as operating leases under prior GAAP, may be classified as a sales-type lease under the new standard. We do not expect the adoption of the new standard will have a significant impact on our leasing activities. For additional information on the Company’s leases, see Note 5 to the condensed consolidated financial statements.
Use of estimates	The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements. Estimates also affect the reported amounts of income and expenses during the reporting period. Actual results could differ from those estimates.
Fair value of financial instruments	The Company believes that the fair values of its financial instruments, including cash and cash equivalents, receivables and payables, approximate their respective book values because of their short-term nature. The fair values described herein were determined using significant other observable inputs (Level 2) as defined by GAAP.

- Definition

Disclosure of accounting policy for environmental remediation costs that are expensed during the period that resulted from improper or other than normal operation of a long-lived asset. This accounting policy may address (1) whether the obligation is measured on a discounted basis, (2) the event, situation, or set of circumstances that generally triggers recognition of loss contingencies arising from the entity's environmental remediation-related obligations, and (3) the timing of recognition of any recoveries. This accounting policy does not address: (1) accounting for pollution control costs of current operations or for costs of future site restoration or closure that are required upon the cessation of operations or sale of facilities (2) environmental remediation actions that are undertaken at the sole discretion of management and that are not induced by the threat, by governments or other parties, of litigation or of assertion of a claim or an assessment (3) recognition of liabilities of insurance companies for unpaid claims or (4) asset impairment issues.

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