Stock Options and Other Equity-Based Incentive Instruments - Information about Options (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Information about Options
Intrinsic value of options exercised	$ 2,657,000	$ 3,483,000	$ 969,000
Fair value of options that vested	259,000	580,000	734,000
Total compensation costs for options recognized in income	259,000	580,000	734,000
Excess tax benefit recognized in income	109,000	$ 244,000	$ 380,000
ASU 2016-09 [Member]
Information about Options
Excess tax benefit recognized in income	$ 906,000