Debt - Debt (Details) - Uncommitted credit lines	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Credit lines, maximum borrowing capacity	$ 324,100,000
Credit lines, remaining borrowing capacity	274,400,000
First, second, and fourth quarters
Line of Credit Facility [Line Items]
Credit lines, maximum borrowing capacity	300,000,000
Third quarter
Line of Credit Facility [Line Items]
Credit lines, maximum borrowing capacity	$ 600,000,000

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.

No definition available.

+ Details

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

+ Details

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Period Type:	instant

- Details

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- Details

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- Details

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