Subsequent Events - Additional Information (Detail) $ / shares in Units, $ in Thousands				12 Months Ended
	Feb. 27, 2019 USD ($) a	Jan. 15, 2019 USD ($) $ / shares shares	Jan. 14, 2019 $ / shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares	Nov. 05, 2018 a
Subsequent Event [Line Items]
Dividends per common share | $ / shares				$ 1.475	$ 1.40	$ 1.20
Options to purchase common shares granted | shares				110,000	110,000
Options exercise price per share | $ / shares				$ 40.00	$ 40.00
Grant date fair value of restricted stock | $				$ 0	$ 0	$ 0
Area of land acquired | a							10
Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends per common share | $ / shares			$ 0.40
Date of dividend payable			Mar. 15, 2019
Record date of dividend payable			Feb. 15, 2019
Area of land acquired | a	9
Purchase price of acquired land | $	$ 8,500
Subsequent Event [Member] | Chief Executive Officer [Member]
Subsequent Event [Line Items]
Shares of restricted stock granted | shares		40,000
Options to purchase common shares granted | shares		110,000
Options exercise price per share | $ / shares		$ 53
Options expiration date		Jan. 15, 2029
Vesting period		4 years
Grant date fair value of restricted stock | $		$ 1,918
Grant date fair value of stock options | $		$ 1,345