Commitments and Contingencies - Additional Information (Detail)	9 Months Ended
Sep. 30, 2018 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Letters of credit, collateral for retrospective premiums and retained losses	$ 63,500,000
Commitments to purchase major equipment and investments	$ 143,000,000
Agreement expire description	The agreements expire in years 2018 through 2022 and in 2042.
Current obligation	$ 123,000,000
Purchase obligations for remainder of 2018	10,300,000
Letter of Credit
Commitments and Contingencies Disclosure [Line Items]
Amount drawn under letters of credit	$ 0

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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