Summary of Significant Accounting Policies - Shares Used in Computation of Basic and Diluted Per Share Information (Detail) - shares	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
Weighted Average Shares Outstanding:
Weighted average number of shares outstanding for basic per share calculation	17,749,872	17,740,818	17,728,567	17,643,463	17,597,190	17,588,163	17,566,097	17,485,766	17,715,992	17,559,477	17,405,379
Effect of dilutive potential shares:
Options									0	863	3,663
Nonvested Stock Awards									245,026	122,480	138,489
Weighted Average Number of Shares Outstanding, Diluted (shares)	18,043,297	18,010,522	17,964,387	17,817,880	17,764,066	17,719,473	17,685,147	17,633,549	17,961,018	17,682,820	17,547,531
Antidilutive shares									0	219,855	200,525