Subsequent Events - Additional Information (Detail) - USD ($)				12 Months Ended
	Feb. 08, 2018	Jan. 26, 2018	Jan. 15, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 20, 2018
Subsequent Event [Line Items]
Dividends per common share				$ 1.40	$ 1.20	$ 1.20
Options to purchase common shares granted				110,000
Options exercise price per share				$ 40.00
Grant date fair value of restricted stock				$ 0	$ 0	$ 0
Capital committed for investment				$ 14,328,000	$ 13,554,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends per common share			$ 0.35
Date of dividend payable			Mar. 16, 2018
Record date of dividend payable			Feb. 16, 2018
Subsequent Event [Member] | Chief Executive Officer [Member]
Subsequent Event [Line Items]
Shares of restricted stock granted	40,000
Options to purchase common shares granted	110,000
Options exercise price per share	$ 40
Options expiration date	Feb. 08, 2028
Vesting period	4 years
Grant date fair value of restricted stock	$ 1,397,000
Grant date fair value of stock options	$ 939,000
Subsequent Event [Member] | Greenleaf Essence, LLC [Member]
Subsequent Event [Line Items]
Purchase of noncontrolling interest		$ 539,000
Gain or loss on acquisition of noncontrolling interest		$ 0
Subsequent Event [Member] | Limited Partnership [Member] | Subscription Agreement [Member]
Subsequent Event [Line Items]
Capital committed for investment							$ 5,000,000