Stock-Based Incentive Plan - Compensation Expense Related to Stock Incentive Plans Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation expense:
Total Expense	$ 3,230	$ 2,151	$ 1,474
Tax benefit on nonqualified stock option expense, restricted stock awards and disqualifying dispositions of incentive stock options	1,432	1,135	339
Stock Options [Member]
Stock-based compensation expense:
Total Expense	910	577	528
Restricted Stock Awards [Member]
Stock-based compensation expense:
Total Expense	2,256	1,507	893
Employee Stock Purchase Plan [Member]
Stock-based compensation expense:
Total Expense	$ 64	$ 67	$ 53