Stockholders' Equity - Additional Information (Detail) - USD ($)				1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 01, 2019	Oct. 23, 2018	Mar. 01, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	May 08, 2018	Oct. 25, 2017
Shareholders Equity [Line Items]
Shares authorized										120,000,000	70,000,000
Common stock, shares authorized					100,000,000		100,000,000		100,000,000	100,000,000	60,000,000
Preferred stock, shares authorized					20,000,000		20,000,000		20,000,000	20,000,000	10,000,000
Payment of regular quarterly dividend							$ 43,678,000	$ 38,761,000
Regular quarterly dividends paid per share							$ 1.05	$ 0.96
Shares issued pursuant to dividend reinvestment, shares							35,424	40,323
Proceeds from issuance of common stock under the dividend reinvestment plan							$ 1,082,000	$ 1,080,000
Stock based compensation expense							$ 1,803,000	$ 1,309,000
Common Stock [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to ATM Program, net, shares					70,000	248,000	283,000	785,000
Shares issued pursuant to dividend reinvestment, shares					12,000	12,000	36,000	41,000
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							$ 8,817,000	$ 20,652,000
ATM Program [Member] | Common Stock [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to ATM Program, net, shares					70,000	248,000	283,000	785,000
Proceeds from issuance of shares					$ 2,154,000	$ 6,938,000	$ 8,817,000	$ 20,652,000
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