Shareholders' Equity - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Aug. 31, 2015	May 31, 2008
Stockholders Equity Note [Abstract]
Common stock shares authorized for repurchase			2,000,000	2,000,000
Shares remain authorized for repurchase	1,592,026
Common stock repurchased, Shares	0	0