Related Party Transactions - Additional Information (Detail) - $ / shares	Sep. 14, 2018	Aug. 23, 2018	Aug. 07, 2018	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			17,141					7,877
Shares price per share			$ 87.19					$ 80.20
Number of trading days to be considered for average price of stock repurchased			30 days					30 days
Officer [Member]
Related Party Transaction [Line Items]
Purchase of shares	7,500			1,500		3,200	1,430
Shares price per share	$ 80.95			$ 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	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
Employee [Member]
Related Party Transaction [Line Items]
Purchase of shares		2,000
Shares price per share		$ 83.53
Number of trading days to be considered for average price of stock repurchased		30 days