Summary of Significant Accounting Policies - Weighted-Average Common Stock Used to Calculate Basic and Diluted Earnings Per Share (Detail) - shares shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Weighted-average common stock for calculating basic earnings per share	24,044	24,015	23,985	23,950	23,927	23,911	23,900	23,862	23,999	23,900	25,369
Effect of potentially dilutive securities from equity-based compensation									270	76	88
Weighted-average common stock for calculating diluted earnings per share	24,410	24,228	24,092	24,232	24,123	24,041	23,949	23,911	24,269	23,976	25,457