Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
Dec. 31, 2018 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Minimum gross proceeds from public offering to reimburse expense	$ 250,000,000

Minimum gross proceeds from public offering to reimburse expense.

No definition available.

Name:	cik0001690536_MinimumGrossProceedsFromPublicOfferingToReimburseExpense
Namespace Prefix:	cik0001690536_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration