Face (par) amount of debt instrument at time of issuance.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount of net gains or losses on interest rate cash flow hedges reclassified during the period to earnings from accumulated other comprehensive income upon the hedged transaction affecting earnings.

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The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

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- 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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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Debt and Short-Term Borrowings - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 31, 2016	May 31, 2016	Apr. 17, 2013
Debt Instrument [Line Items]
Losses reclassified from accumulated other comprehensive loss into income	$ 100,000
Gains expected to be reclassified from accumulated other comprehensive loss into income in the next 12 months	500,000
Notes Payable
Debt Instrument [Line Items]
Non interest bearing financing agreement				$ 1,100,000.0	$ 700,000.0
Note payable	400,000	$ 1,000,000
Term A due on April 17, 2018 \| Credit Facility
Debt Instrument [Line Items]
Maximum amount under credit facilities			$ 250,000,000			$ 300,000,000.0
Term A due on April 17, 2018 \| Credit Facility \| Maturity not extended
Debt Instrument [Line Items]
Maximum amount under credit facilities			30,000,000
2013 Credit Agreement, Term B Loan \| Credit Facility
Debt Instrument [Line Items]
Maximum amount under credit facilities						400,000,000.0
2013 Credit Agreement, Revolving Facility \| Credit Facility
Debt Instrument [Line Items]
Maximum amount under credit facilities						$ 100,000,000.0
Term A due on January 17, 2020 \| Credit Facility
Debt Instrument [Line Items]
Secured credit facilities	191,400,000
Term A due on January 17, 2020 \| Credit Facility \| Maturity extended
Debt Instrument [Line Items]
Maximum amount under credit facilities			219,000,000
Term B due on April 17, 2020 \| Credit Facility
Debt Instrument [Line Items]
Secured credit facilities	379,000,000
Senior Secured Revolving Credit Facility \| Credit Facility
Debt Instrument [Line Items]
Maximum amount under credit facilities			100,000,000
Additional borrowing capacity available under the Revolving Facility	$ 65,000,000
Senior Secured Revolving Credit Facility \| Credit Facility \| Maturity extended
Debt Instrument [Line Items]
Maximum amount under credit facilities			65,000,000
Senior Secured Revolving Credit Facility \| Credit Facility \| Maturity not extended
Debt Instrument [Line Items]
Maximum amount under credit facilities			$ 35,000,000