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Long-term Debt (Narrative) (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Term of credit facility, years	5 years
Revolving credit facility, maturity date	Sep. 22, 2022
Average interest rate during the period	3.00%	2.50%
4.10% Senior Notes [Member]
Debt Instrument [Line Items]
Senior notes, interest rate	4.10%
Principal amount of debt	$ 350,000,000
3.78% Senior Notes [Member]
Debt Instrument [Line Items]
Senior notes, interest rate	3.78%		3.78%

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- Definition

Term of credit facility

+ References

No definition available.

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X

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

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-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

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.

+ References

No definition available.

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- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

The effective interest rate during the reporting period.

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-SubTopic 10
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- Details

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