Summary of Significant Accounting Policies - Goodwill (Details) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill impairment	$ 0	$ 0	$ 0	$ 0
Impairment of long-lived assets		$ 0	$ 0	$ 0

Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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