Stockholders' Equity - Additional Information (Detail)					1 Months Ended		12 Months Ended
	Oct. 23, 2018 shares	Mar. 01, 2018 shares	Jul. 10, 2017 USD ($) Offering shares	Mar. 01, 2017 shares	Mar. 31, 2018 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 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	10,000,000		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	60,000,000		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 dividend | $							$ 50,503,000	$ 39,299,000
Dividends paid per share | $ / shares							$ 1.28	$ 1.12
Proceeds from issuance of common stock under the dividend reinvestment plan | $							$ 1,403,000	$ 1,271,000
Stock based compensation expense | $							$ 1,777,000	$ 1,350,000	$ 1,426,000
Common Stock [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to dividend reinvestment, shares							51,920	47,922	43,000
2016 ATM Program [Member] | Maximum [Member]
Shareholders Equity [Line Items]
Aggregate sales price | $						$ 125,000,000
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 | $							$ 30,138,000	$ 13,528,000	$ 14,886,000
ATM Program [Member] | Common Stock [Member]
Shareholders Equity [Line Items]
Proceeds from issuance of shares | $							$ 30,138,000	$ 13,528,000
Total shares issued							1,106,000	513,000	653,000
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							124,650	94,250	86,600
Amended and Restated 2004 Omnibus Incentive Compensation Plan [Member] | Restricted Stock Units [Member]
Shareholders Equity [Line Items]
Restricted stock units, granted	3,000	121,650		94,250