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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Name:	us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Summary of Significant Accounting Policies - Trade Accounts Receivable (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts Notes And Loans Receivable [Line Items]
Bad debt expense, net of recoveries		$ 0.2	$ 0.3
Maximum [Member]
Accounts Notes And Loans Receivable [Line Items]
Bad debt recovery, net	$ 0.1