Stockholders' Equity - Summary of Common Stock Transactions (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Balance at beginning of period, shares			0
Balance at beginning of period, amount	$ 29,339	$ 200
Issuance of common stock, amount	52,627	28,998	$ 200
Reinvestment of distributions, amount	2,459	328
Redemptions of common stock, amount	(100)
Balance at end of period, amount	$ 83,436	$ 29,339	$ 200
Common Stock [Member]
Class of Stock [Line Items]
Balance at beginning of period, shares	1,179,915	8,000
Issuance of common stock, shares	2,124,933	1,158,819
Reinvestment of distributions, shares	99,814	13,096
Redemptions of common stock, shares	(3,979)
Balance at end of period, shares	3,400,683	1,179,915	8,000
Balance at beginning of period, amount	$ 29,526	$ 200
Issuance of common stock, amount	52,627	28,998
Reinvestment of distributions, amount	2,459	328
Redemptions of common stock, amount	(100)
Accrued stockholder servicing fees, amount	(923)
Balance at end of period, amount	$ 83,589	$ 29,526	$ 200
Common Stock [Member] | Common Class F [Member]
Class of Stock [Line Items]
Balance at beginning of period, shares	988,801	8,000
Issuance of common stock, shares	1,387,498	968,796
Reinvestment of distributions, shares	95,565	12,005
Balance at end of period, shares	2,471,864	988,801	8,000
Balance at beginning of period, amount	$ 24,741	$ 200
Issuance of common stock, amount	34,175	24,240
Reinvestment of distributions, amount	2,353	301
Balance at end of period, amount	$ 61,269	$ 24,741	$ 200
Common Stock [Member] | Common Class Y [Member]
Class of Stock [Line Items]
Balance at beginning of period, shares	191,114
Issuance of common stock, shares		190,023
Reinvestment of distributions, shares	1,899	1,091
Balance at end of period, shares	193,013	191,114
Balance at beginning of period, amount	$ 4,785
Issuance of common stock, amount		$ 4,758
Reinvestment of distributions, amount	47	27
Balance at end of period, amount	$ 4,832	$ 4,785
Common Stock [Member] | Common Class T [Member]
Class of Stock [Line Items]
Issuance of common stock, shares	123,923
Reinvestment of distributions, shares	658
Balance at end of period, shares	124,581
Issuance of common stock, amount	$ 3,104
Reinvestment of distributions, amount	16
Accrued stockholder servicing fees, amount	(133)
Balance at end of period, amount	$ 2,987
Common Stock [Member] | Common Class S [Member]
Class of Stock [Line Items]
Issuance of common stock, shares	3,752
Reinvestment of distributions, shares	21
Balance at end of period, shares	3,773
Issuance of common stock, amount	$ 94
Reinvestment of distributions, amount	1
Accrued stockholder servicing fees, amount	(4)
Balance at end of period, amount	$ 91
Common Stock [Member] | Common Class D [Member]
Class of Stock [Line Items]
Issuance of common stock, shares	60,774
Reinvestment of distributions, shares	160
Balance at end of period, shares	60,934
Issuance of common stock, amount	$ 1,522
Reinvestment of distributions, amount	4
Accrued stockholder servicing fees, amount	(19)
Balance at end of period, amount	$ 1,507
Common Stock [Member] | Common Class M [Member]
Class of Stock [Line Items]
Issuance of common stock, shares	421,104
Reinvestment of distributions, shares	867
Redemptions of common stock, shares	(3,979)
Balance at end of period, shares	417,992
Issuance of common stock, amount	$ 10,581
Reinvestment of distributions, amount	22
Redemptions of common stock, amount	(100)
Accrued stockholder servicing fees, amount	(767)
Balance at end of period, amount	$ 9,736
Common Stock [Member] | Common Class I [Member]
Class of Stock [Line Items]
Issuance of common stock, shares	127,882
Reinvestment of distributions, shares	644
Balance at end of period, shares	128,526
Issuance of common stock, amount	$ 3,151
Reinvestment of distributions, amount	16
Balance at end of period, amount	$ 3,167