Shareholders' Equity - Additional Information (Detail)			1 Months Ended		3 Months Ended		6 Months Ended
	Mar. 01, 2018 shares	Jul. 10, 2017 USD ($) Offering shares	Mar. 31, 2018 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) shares	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	May 08, 2018 shares	Dec. 31, 2017 $ / shares 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	10,000,000
Preferred stock, par value | $ / shares					$ 0.01		$ 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		$ 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	60,000,000
Proceeds from issuance of shares | $							$ 13,714,000	$ 5,860,000
Payment of regular quarterly dividend | $							$ 25,736,000	$ 19,569,000
Regular quarterly dividends paid per share | $ / shares							$ 0.64	$ 0.56
Shares issued pursuant to dividend reinvestment, shares							28,884	24,284
Shares issued pursuant to dividend reinvestment, value | $							$ 755,000	$ 628,000
Stock based compensation expenses | $							$ 848,000	655,000
Common Stock [Member]
Shareholders Equity [Line Items]
Total shares issued							537,000
Shares issued pursuant to dividend reinvestment, shares							29,000
2016 ATM Program [Member] | Common Stock [Member]
Shareholders Equity [Line Items]
Proceeds from issuance of shares | $						$ 1,580,000		$ 5,860,000
Total shares issued						64,000		232,000
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] | Common Stock [Member]
Shareholders Equity [Line Items]
Proceeds from issuance of shares | $					$ 13,714,000		$ 13,714,000
Total shares issued					537,000		537,000
Underwriting Agreement [Member]
Shareholders Equity [Line Items]
Number Of Equity Offering Completed | Offering		1
Number of shares of common stock the Company sold to the underwriters pursuant to the underwriting agreement		4,100,000
Period granted to underwriter for purchase of additional shares		30 days
Common stock offering underwriting agreement additional shares granted		615,000
Proceeds from issuance of shares | $		$ 104,312,000
Amended and Restated 2004 Omnibus Incentive Compensation Plan [Member] | Restricted Stock Units [Member]
Shareholders Equity [Line Items]
Restricted stock units, granted	121,650