Date the credit agreement expires.

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Credit Agreement, Initiation Date

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No definition available.

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- Definition

Amount of increase in borrowing capacity if additional lenders are identified and/or existing lenders are willing to increase their current commitments.

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No definition available.

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- Definition

Letters of credit maximum borrowing capacity.

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No definition available.

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Range of letter of credit fees.

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No definition available.

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- Definition

Amount of the total principal payments made during the annual reporting period.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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No definition available.

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Amount of the required periodic payments applied to principal.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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No definition available.

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The effective interest rate at the end of the reporting period.

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Description of the interest rate for the amounts borrowed under the credit facility, including the terms and the method for determining the interest rate (for example, fixed or variable, LIBOR plus a percentage, increasing rate, timing of interest rate resets, remarketing provisions).

+ References

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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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No definition available.

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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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- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

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No definition available.

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The interest rate applicable to the portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a rate subject to change from time to time.

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The carrying value as of the balance sheet date of the noncurrent portion 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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The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

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Debt (Narrative) (Details) - USD ($)		3 Months Ended		6 Months Ended		12 Months Ended
Debt (Narrative) (Details) - USD ($)	Aug. 03, 2016	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 30, 2016
Line of Credit Facility [Line Items]
Interest rate description				Borrowings under the 2016 Credit Agreement bear interest at (1) a base rate plus a margin of 0.00% to 0.75%, based on certain financial tests, or (2) United States dollar LIBOR (2.09% and 2.33% at June 30, 2018 for our 2016 Revolving Credit Facility and our 2016 Term Loan, respectively) plus 1.00% to 1.75%, based on certain financial tests. The base rate is determined by the greater of (a) the prime commercial lending rate announced by Bank of Montreal from time to time (5.00% at June 30, 2018), (b) the federal funds effective rate, plus ½ of 1.00%, (c) the daily one month LIBOR rate, plus 1.00%, or (d) 0.00%. The interest rates in effect at June 30, 2018 were 3.09% and 3.33% for our 2016 Revolving Credit Facility and our 2016 Term Loan, respectively. Fees for letters of credit issued under the 2016 Revolving Credit Facility range from 1.00% to 1.75% of the respective face amounts of outstanding letters of credit and are computed based on certain financial tests.
Repayments of long-term debt (in US dollars)				$ 7,634,000	$ 7,601,000
Letters of credit outstanding (in US dollars)				110,300,000			$ 110,100,000
Borrowings under revolving credit facility (in US dollars)				$ 25,000,000			25,000,000
2016 Credit Agreement
Line of Credit Facility [Line Items]
Credit agreement, initiation date	Aug. 03, 2016
Expiration date of credit agreement				Aug. 03, 2021
2016 Revolving Credit Facility
Line of Credit Facility [Line Items]
Interest rate				3.09%
Borrowings under revolving credit facility (in US dollars)				$ 25,000,000			25,000,000
2016 Term Loan
Line of Credit Facility [Line Items]
Term Loan (in US dollars)	$ 400,000,000			$ 277,215,000			$ 284,810,000
Interest rate				3.33%
Periodic principal payment on term loan (in US dollars)		$ 3,800,000	$ 5,000,000			$ 5,000,000
Term loan, annual principal payments (in US dollars)								$ 100,000,000
Repayments of long-term debt (in US dollars)						$ 95,000,000
Base Rate [Member] \| Minimum [Member] \| 2016 Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate				0.00%
Base Rate [Member] \| Maximum [Member] \| 2016 Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate				0.75%
Prime Rate, Bank of Montreal [Member] \| 2016 Credit Agreement
Line of Credit Facility [Line Items]
Commercial lending rate				5.00%
Credit Agreement Base Rate, Daily One Month LIBOR Rate [Member] \| 2016 Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate				1.00%
London Interbank Offered Rate (LIBOR) [Member] \| 2016 Revolving Credit Facility
Line of Credit Facility [Line Items]
Commercial lending rate				2.09%
London Interbank Offered Rate (LIBOR) [Member] \| 2016 Term Loan
Line of Credit Facility [Line Items]
Commercial lending rate				2.33%
London Interbank Offered Rate (LIBOR) [Member] \| Minimum [Member] \| 2016 Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate				1.00%
London Interbank Offered Rate (LIBOR) [Member] \| Maximum [Member] \| 2016 Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate				1.75%
Credit Agreement Base Rate, Federal Funds Rate [Member] \| 2016 Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate				0.50%
Credit Agreement, 0% Base Rate [Member] \| 2016 Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate				0.00%
Revolving Credit Facility [Member] \| 2016 Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity (in US dollars)	$ 900,000,000
Increase in borrowing capacity (in US dollars)				$ 1,300,000,000
Letters of credit maximum borrowing capacity (in US dollars)				$ 300,000,000
Commitment fee percentage of unused amount				0.15%
Revolving Credit Facility [Member] \| Minimum [Member] \| 2016 Revolving Credit Facility
Line of Credit Facility [Line Items]
Commitment fee percentage of unused amount				0.15%
Letter of credit fees				1.00%
Revolving Credit Facility [Member] \| Maximum [Member] \| 2016 Revolving Credit Facility
Line of Credit Facility [Line Items]
Commitment fee percentage of unused amount				0.30%
Letter of credit fees				1.75%