Stockholders' Equity - Additional Information (Detail)					1 Months Ended	12 Months Ended
	Mar. 01, 2019 shares	Oct. 23, 2018 shares	Mar. 01, 2018 shares	Jul. 10, 2017 USD ($) Offering shares	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	May 08, 2018 shares	Oct. 25, 2017 shares	Oct. 24, 2017 $ / shares shares
Shareholders Equity [Line Items]
Preferred stock, shares authorized						20,000,000	20,000,000		20,000,000	10,000,000	10,000,000
Preferred stock, par value | $ / shares						$ 0.01	$ 0.01				$ 0.01
Authorized but unissued preferred stock reclassified to common stock											10,000,000
Common stock, par value | $ / shares						$ 0.01	$ 0.01				$ 0.01
Shares authorized									120,000,000	70,000,000
Common stock, shares authorized						100,000,000	100,000,000		100,000,000	60,000,000
Number of shares of common stock the Company sold to the underwriters pursuant to the underwriting agreement				4,100,000
Common stock offering underwriting agreement additional shares granted				615,000
Payment of regular quarterly dividend | $						$ 56,889,000	$ 50,503,000
Regular quarterly dividends paid per share | $ / shares						$ 1.40	$ 1.28
Shares issued pursuant to dividend reinvestment, shares						46,896	51,920
Proceeds from issuance of common stock under the dividend reinvestment plan | $						$ 1,451,000	$ 1,403,000
Stock based compensation expense | $						$ 2,468,000	$ 1,777,000	$ 1,350,000
Common Stock [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to dividend reinvestment, shares						47,000	52,000	48,000
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 | $						$ 14,150,000	$ 30,138,000	$ 13,528,000
ATM Program [Member] | Common Stock [Member]
Shareholders Equity [Line Items]
Proceeds from issuance of shares | $						$ 14,150,000	$ 30,138,000
Shares issued pursuant to Offering/Program net, shares						449,000	1,106,000	513,000
Underwriting Agreement [Member]
Shareholders Equity [Line Items]
Number Of Equity Offering Completed | Offering				1
Period granted to underwriter for purchase of additional shares				30 days
Proceeds from issuance of shares | $				$ 104,312,000
Restricted Stock Units [Member]
Shareholders Equity [Line Items]
Restricted stock units, granted						156,750	124,650	94,250
Amended and Restated 2004 Omnibus Incentive Compensation Plan [Member] | Restricted Stock Units [Member]
Shareholders Equity [Line Items]
Restricted stock units, granted	156,750	3,000	121,650