Stock-Based Compensation - Performance-Based Restricted Stock Activity (Detail) - Performance-Based Restricted Stock and Performance-Based Restricted Stock Units [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Restricted Stock Activity [Line Items]
Nonvested, Shares, Beginning Balance	58,416	29,097	42,676
Performance-based Restricted Stock, Granted	43,307	44,925	34,638
Performance-based Restricted Stock, Vested	(43,394)	(14,949)	(35,679)
Performance-based Restricted Stock, Forfeited		(657)	(12,538)
Nonvested, Shares, Ending Balance	58,329	58,416	29,097
Nonvested, Weighted-Average Grant Price Per Share, Beginning Balance	$ 36.63	$ 39.38	$ 33.72
Weighted-Average Grant Price Per Share, Granted	57.54	35.83	43.89
Weighted-Average Grant Price Per Share, Vested	37.27	35.71	41.91
Weighted-Average Grant Price Per Share, Forfeited		33.72	38.12
Nonvested, Weighted-Average Grant Price Per Share, Ending Balance	$ 51.69	$ 36.63	$ 39.38