Stockholders' Equity - Additional Information (Detail) - USD ($)			1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 02, 2020	Mar. 01, 2019	Mar. 31, 2018	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Shareholders Equity [Line Items]
Payment of regular quarterly dividend						$ 31,189,000	$ 29,050,000
Regular quarterly dividends paid per share						$ 0.74	$ 0.70
Shares issued pursuant to dividend reinvestment, shares						13,027	23,571
Proceeds from issuance of common stock under the dividend reinvestment plan						$ 410,000	$ 718,000
Stock based compensation expense						$ 1,600,000	$ 1,139,000
Common Stock [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to dividend reinvestment, shares				1,000	12,000	13,000	24,000
2016 ATM Program [Member] | Maximum [Member]
Shareholders Equity [Line Items]
Aggregate sales price			$ 125,000,000
ATM Program [Member]
Shareholders Equity [Line Items]
Proceeds from issuance of shares						$ 11,916,000	$ 6,663,000
ATM Program [Member] | Common Stock [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, shares				404,000	213,000	404,000	213,000
Proceeds from issuance of shares				$ 11,916,000	$ 6,663,000	$ 11,916,000	$ 6,663,000
Amended and Restated 2004 Omnibus Incentive Compensation Plan [Member] | Restricted Stock Units [Member]
Shareholders Equity [Line Items]
Restricted stock units, granted	176,050	156,750