Shareholders' Equity - Additional Information (Detail)			1 Months Ended	3 Months Ended	9 Months Ended		12 Months Ended
	Jul. 10, 2017 USD ($) Offering shares	Mar. 01, 2017 shares	Jun. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares shares
Shareholders Equity [Line Items]
Stock options outstanding | shares				0	0		5,000
Stock options exercisable price | $ / shares				$ 27.68	$ 27.68		$ 27.68
Stock options expiration date					May 15, 2017		May 15, 2017
Stock options outstanding intrinsic value				$ 0	$ 0		$ 0
Preferred stock, shares authorized | shares				20,000,000	20,000,000		20,000,000
Preferred stock, par value | $ / shares				$ 0.01	$ 0.01		$ 0.01
Preferred stock, shares issued | shares				0	0		0
Proceeds from issuance of shares					$ 112,857,000	$ 2,940,000
Payment of regular quarterly dividend					$ 29,404,000	$ 25,532,000
Regular quarterly dividends paid per share | $ / shares					$ 0.84	$ 0.75
Payments of regular and special dividends						$ 32,973,000
Dividends paid per share | $ / shares						$ 0.97
Payment of special dividend						$ 7,441,000
Special dividends paid per share | $ / shares						$ 0.22
Shares issued under dividend reinvestment plan | shares					36,865
Value of shares issued under Shares issued under Dividend Reinvestment Plan					$ 947,000
Stock based compensation expenses					996,000	$ 1,153,000
ATM Program [Member]
Shareholders Equity [Line Items]
Proceeds from issuance of shares				$ 2,685,000	$ 8,545,000
Stock issuance costs			$ 360,000
Shares issued | shares				97,000	329,000
ATM Program [Member] | Maximum [Member]
Shareholders Equity [Line Items]
Aggregate sales price			$ 125,000,000
Underwriting Agreement [Member]
Shareholders Equity [Line Items]
Number of shares of common stock the Company sold to the underwriters pursuant to the underwriting agreement | shares	4,100,000
Period granted to underwriter for purchase of additional shares	30 days
Common stock offering underwriting agreement additional shares granted | shares	615,000
Proceeds from issuance of shares	$ 104,300,000
Number Of Equity Offering Completed | Offering	1
Amended and Restated 2004 Omnibus Incentive Compensation Plan [Member] | Restricted Stock Units [Member]
Shareholders Equity [Line Items]
Restricted stock units, granted | shares		94,250