Related Party Transactions - Additional Information (Detail) - $ / shares	Jun. 15, 2018	Jun. 01, 2018	May 10, 2018	Mar. 15, 2018	Feb. 06, 2018
Current Officers and Retired Officer [Member]
Related Party Transaction [Line Items]
Purchase of shares					7,877
Shares price per share					$ 80.20
Number of trading days to be considered for average price of stock repurchased					30 days
Officer [Member]
Related Party Transaction [Line Items]
Purchase of shares	1,500		3,200	1,430
Shares price per share	$ 77.06		$ 68.10	$ 63.63
Number of trading days to be considered for average price of stock repurchased	30 days		30 days	30 days
Retired Officer [Member]
Related Party Transaction [Line Items]
Purchase of shares		8,800
Shares price per share		$ 73.53
Number of trading days to be considered for average price of stock repurchased		30 days