Revenue from Contracts with Customers (Narrative) (Details)	3 Months Ended
Mar. 31, 2018 USD ($) contract item segment agreement	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Disaggregation of Revenue [Line Items]
Number of geographic segments \| segment	3
Number of revenue streams \| item	3
Receivables from contracts with customers	$ 178,700,000		$ 203,400,000
Impairment losses on receivables or contract assets	0	$ 0
Revenue, remaining performance obligation	$ 0
Malaysia [Member]
Disaggregation of Revenue [Line Items]
Number of production sharing contracts \| contract	9
Number of gas sale agreements \| agreement	3

Number of Gas Sale Agreements

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- Definition

Number of Geographic Segments

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- Definition

Number of Production Sharing Contracts

+ References

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- Definition

Number of Revenue Streams

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- Definition

Amount of credit loss recognized for write-down of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Topic 606
-SubTopic 10
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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.

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Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

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