Revenue - Receivables (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total customer receivables	$ 331,601	$ 297,796
Billed receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total customer receivables	318,305	285,800
Unbilled receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total customer receivables	$ 13,296	$ 11,996

Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

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 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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