Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and restricted cash	$ 36.1	$ 20.7	$ 5.8

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

No definition available.

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