Financing Arrangements - Additional Information (Detail)			3 Months Ended	12 Months Ended
	Jan. 26, 2018 USD ($) Extension	Aug. 30, 2017 USD ($) Extension	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Feb. 20, 2019 USD ($)	Jun. 30, 2018 USD ($)
Assets Sold under Agreements to Repurchase [Line Items]
Deferred financing costs				$ 1,314,000	$ 658,000
Deferred financing costs			$ 452,000	878,000	$ 452,000
WF-1 Facility [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Average borrowings			$ 23,090,000	$ 106,424,000
Funding period and term of facility				3 years 4 months 24 days	3 years 1 month 6 days
Deferred financing costs		$ 815,000
Deferred financing costs				$ 165,000
Covenant description				The WF-1 Repurchase Agreement and WF-1 Guarantee, as amended, contain representations, warranties, covenants, events of default and indemnities that are customary for agreements of their type. In addition, WF-1 is required to maintain a certain minimum liquidity amount in a collateral account with Wells Fargo and the Company is required (i) to maintain its adjusted tangible net worth at an amount equal to or greater than the greater of (A) the sum of $37,500 plus 75% of all equity capital raised by it from and after the closing date and (B) 75% of the then-current maximum facility size; (ii) to maintain, commencing on September 30, 2018, an earnings before interest, taxes, depreciation and amortization, or EBITDA, to interest expense ratio not less than 1.50 to 1.00; (iii) to maintain a total indebtedness to tangible net worth ratio of less than 3.00 to 1.00; and (iv) to maintain liquidity of not less than 7.5% of the amount outstanding under the WF-1 Facility.
Equity capital threshold, amount				$ 37,500,000
Equity capital threshold, percentage				75.00%
Percentage of current maximum facility				75.00%
EBITDA to interest expense ratio				150.00%
Total indebtedness to tangible net worth ratio				300.00%
Cash to be maintained in cash collateral accounts with respect to maximum borrowings				7.50%
Weighted Average Rate			3.75%	4.49%	3.75%
WF-1 Facility [Member] | FS CREIT Finance WF -1 LLC [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Maximum amount of financing available		75,000,000
Increase in amount of financing available		$ 150,000,000
Funding period and term of facility		1 year
Number of extensions of funding period | Extension		3
Extension period of funding		1 year
WF-1 Facility [Member] | Minimum [Member] | FS CREIT Finance WF -1 LLC [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Increase in amount of financing available							$ 150,000,000
WF-1 Facility [Member] | Maximum [Member] | FS CREIT Finance WF -1 LLC [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Increase in amount of financing available				$ 200,000,000
GS-1 Facility [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Funding period and term of facility				3 years 9 months 18 days
Deferred financing costs				$ 1,157,000
Deferred financing costs				$ 713,000
Covenant description				The GS-1 Repurchase Agreement and GS-1 Guarantee contain representations, warranties, covenants, events of default and indemnities that are customary for agreements of their type. In addition, the Company is required (i) to maintain its adjusted tangible net worth at an amount equal to or greater than $37,500 plus 75% of all equity capital raised by the Company from and after the closing date; (ii) to maintain an EBITDA to interest expense ratio not less than 1.50 to 1.00; (iii) to maintain a total indebtedness to tangible net worth ratio of less than 3.00 to 1.00; and (iv) to maintain liquidity at not less than 7.5% of the amount outstanding under the GS-1 Facility.
Equity capital threshold, amount	$ 37,500,000
Equity capital threshold, percentage	75.00%
EBITDA to interest expense ratio	150.00%
Total indebtedness to tangible net worth ratio	300.00%
Weighted Average Rate				4.35%
GS-1 Facility [Member] | FS CREIT Finance WF -1 LLC [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Maximum amount of financing available	$ 130,000,000
Funding period and term of facility	2 years
Number of extensions of funding period | Extension	2
Extension period of funding	1 year
Guarantee obligations percentage of repurchase agreement	50.00%
Increase in amount of financing available upon achivement of milestone	$ 250,000,000
GS-1 Facility [Member] | Maximum [Member] | FS CREIT Finance WF -1 LLC [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Increase in amount of financing available				$ 130,000,000
GS-1 Facility [Member] | Maximum [Member] | FS CREIT Finance WF -1 LLC [Member] | Subsequent Event [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Increase in amount of financing available						$ 175,000,000