Related Party Transactions - Summary of Fees and Expenses Incurred and Amounts of Investment Funding Due (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Fees and expenses, Due to related parties	$ 1,816,091	$ 1,438,450	$ 2,057,181
Fees and expenses, Incurred	2,695,312	2,719,182
Fees and expenses, Paid	3,121,671	3,337,913
Asset Management Fees [Member]
Related Party Transaction [Line Items]
Fees and expenses, Due to related parties	197,776	150,028	123,179
Fees and expenses, Incurred	1,524,315	1,663,624
Fees and expenses, Paid	1,476,567	1,636,775
Property Management and Oversight Fees [Member]
Related Party Transaction [Line Items]
Fees and expenses, Due to related parties	57,924	37,055	20,269
Fees and expenses, Incurred	397,364	145,547
Fees and expenses, Paid	376,495	128,761
Operating Expenses [Member]
Related Party Transaction [Line Items]
Fees and expenses, Due to related parties	204,253	204,253	204,253
Organization Expenses [Member]
Related Party Transaction [Line Items]
Fees and expenses, Due to related parties		17,879	71,162
Fees and expenses, Incurred		444
Fees and expenses, Paid	17,879	53,727
Admin Fees
Related Party Transaction [Line Items]
Fees and expenses, Due to related parties	12,600
Fees and expenses, Incurred	12,600
Offering Costs [Member]
Related Party Transaction [Line Items]
Fees and expenses, Due to related parties	68,260	294,405	718,499
Fees and expenses, Incurred	500,896	264,083
Fees and expenses, Paid	727,041	688,177
Selling Commissions and Dealer Manager Fees, Net [Member]
Related Party Transaction [Line Items]
Fees and expenses, Incurred	265,498	528,442
Fees and expenses, Paid	265,498	528,442
Distribution Fees [Member]
Related Party Transaction [Line Items]
Fees and expenses, Due to related parties	471,278	734,830	$ 919,819
Fees and expenses, Incurred	(5,361)	117,042
Fees and expenses, Paid	258,191	$ 302,031
Distribution Due [Member]
Related Party Transaction [Line Items]
Fees and expenses, Due to related parties	84,000
Demand Note [Member]
Related Party Transaction [Line Items]
Fees and expenses, Due to related parties	$ 720,000