Shareholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended		3 Months Ended	6 Months Ended
	Jul. 05, 2017	Jul. 31, 2017	Mar. 31, 2017	Jun. 30, 2019	Jun. 30, 2019	Mar. 14, 2019	Dec. 31, 2018	Aug. 31, 2015
Schedule Of Capitalization Equity [Line Items]
Preferred stock, par value				$ 0.01	$ 0.01		$ 0.01
Reimbursement paid for every $100 of performance incentive fees earned					$ 10
Performance incentive fees earned					100
Payments of contingent underwriting fees to underwriters				$ 201,000	353,000
IPO [Member]
Schedule Of Capitalization Equity [Line Items]
Reimbursement paid for every $100 of performance incentive fees earned					20
Performance incentive fees earned					$ 100
Reimbursement agreement expiry date					Feb. 01, 2023
Underwriting fees waived				1,100,000
Management [Member]
Schedule Of Capitalization Equity [Line Items]
Reimbursement of contingent underwriting fee to manager				75,000	$ 75,000
Maximum [Member]
Schedule Of Capitalization Equity [Line Items]
Common stock shares Repurchase authorized amount								$ 300,000,000
Equity Distribution Agreement [Member]
Schedule Of Capitalization Equity [Line Items]
Maximum aggregate offering price						$ 200,000,000
Common stock shares available for issuance value				$ 151,100,000	$ 151,100,000
8.125% Series A Preferred Stock [Member]
Schedule Of Capitalization Equity [Line Items]
Issuance of preferred shares, Shares			4,600,000
Cumulative dividend, beneficial interest rate			8.125%		8.125%
Preferred stock, par value			$ 0.01	$ 0.01	$ 0.01
Statutory Dividend Payment Restrictions Disclosure					The Company pays quarterly cumulative dividends on the Preferred Shares on the 15th day of each March, June, September and December, provided that if any dividend payment date is not a business day, then the dividend that would otherwise be payable on that dividend payment date may be paid on the following business day.
8.125% Series A Preferred Stock [Member] | Fixed Annuity | Date Of Original Issuance To March 14,2024 [Member]
Schedule Of Capitalization Equity [Line Items]
Preferred stock, liquidation preference			$ 25.00
8.125% Series A Preferred Stock [Member] | Floating Rate [Member] | March 15,2024 and Thereafter [member]
Schedule Of Capitalization Equity [Line Items]
Cumulative dividend, beneficial interest rate			5.831%
8.00% Series B Preferred Stock [Member]
Schedule Of Capitalization Equity [Line Items]
Issuance of preferred shares, Shares		7,800,000
Cumulative dividend, beneficial interest rate	8.00%	5.99%			8.00%
Preferred stock, par value		$ 0.01		0.01	$ 0.01
Preferred stock, liquidation preference		$ 25.00
8.125% Series A And 8.00% Series B Preferred Stock [Member]
Schedule Of Capitalization Equity [Line Items]
Preferred stock redemption price per share				$ 25.00	$ 25.00