Summary of Significant Accounting Policies - Basic and Diluted Shares Outstanding (Detail) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average common shares outstanding - basic (in shares)	21,901	21,505	21,127
Effect of dilutive options (in shares)	107	110	69
Weighted average common shares outstanding - diluted (in shares)	22,008	21,615	21,196
Average share price (in usd per share)	$ 61.63	$ 58.96	$ 53.38
Options
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive options (in shares)	0	129	111