Revenues - Contract Assets (Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from Contract with Customer [Abstract]
Contract assets – current	$ 88,348	$ 75,371
Contract liabilities – current	(35,297)	(51,597)	$ (62,700)
Net contract assets	53,051	23,774
Contract assets, held-for-sale	2,200	1,300
Contract liabilities, held-for-sale	$ 100	$ 5,500

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
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-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130533-203044

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant