Related Party Transactions - Additional Information (Detail)																			12 Months Ended
	Dec. 18, 2018 $ / shares shares	Sep. 14, 2018 $ / shares shares	Aug. 23, 2018 $ / shares shares	Aug. 07, 2018 $ / shares shares	Jun. 15, 2018 $ / shares shares	Jun. 01, 2018 $ / shares shares	May 10, 2018 $ / shares shares	Mar. 15, 2018 $ / shares shares	Feb. 06, 2018 $ / shares shares	Dec. 13, 2017 $ / shares shares	Nov. 30, 2017 $ / shares shares	Nov. 17, 2017 $ / shares shares	Nov. 08, 2017 $ / shares shares	Aug. 16, 2017 $ / shares shares	May 09, 2017 $ / shares shares	Jan. 03, 2017 $ / shares shares	Dec. 29, 2016 $ / shares shares	Aug. 23, 2016 $ / shares shares	Dec. 31, 2018 USD ($) Lease	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) Lease $ / shares shares
Related Party Transaction [Line Items]
Related party transactions number of property leased | Lease																			1		2
Related party transaction lease expenses | $																			$ 0		$ 300,000
Maximum [Member]
Related Party Transaction [Line Items]
Related party transaction lease expenses | $																				$ 100,000
Officers and Other Employees [Member]
Related Party Transaction [Line Items]
Purchase of shares | shares										21,650		7,975	24,874								7,703
Shares price per share										$ 78.68		$ 74.51	$ 71.07
Number of trading days to be considered for average price of stock repurchased										30 days		30 days	30 days								30 days
Officers and Other Employees [Member] | Minimum [Member]
Related Party Transaction [Line Items]
Shares price per share																					$ 42.10
Officers and Other Employees [Member] | Maximum [Member]
Related Party Transaction [Line Items]
Shares price per share																					$ 45.20
Retired Officer [Member]
Related Party Transaction [Line Items]
Purchase of shares | shares											3,334
Shares price per share											$ 75.37
Number of trading days to be considered for average price of stock repurchased											30 days
Randall M. Ruhlman [Member]
Related Party Transaction [Line Items]
Purchase of shares | shares										7,500
Shares price per share										$ 78.68
Number of trading days to be considered for average price of stock repurchased										30 days
Current Officers and Retired Officer [Member]
Related Party Transaction [Line Items]
Purchase of shares | 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 | shares		7,500			1,500		3,200	1,430							2,500	1,834	5,000
Shares price per share		$ 80.95			$ 77.06		$ 68.10	$ 63.63							$ 52.05	$ 58.58	$ 56.38
Number of trading days to be considered for average price of stock repurchased		30 days			30 days		30 days	30 days							30 days	30 days	30 days
Retired Officer [Member]
Related Party Transaction [Line Items]
Purchase of shares | shares	35					8,800
Shares price per share	$ 57.85					$ 73.53
Number of trading days to be considered for average price of stock repurchased	30 days					30 days
Employee [Member]
Related Party Transaction [Line Items]
Purchase of shares | shares			2,000
Shares price per share			$ 83.53
Number of trading days to be considered for average price of stock repurchased			30 days
Trust [Member]
Related Party Transaction [Line Items]
Purchase of shares | shares										15,000				24,920				27,448
Shares price per share										$ 78.68				$ 50.16				$ 44.66
Number of trading days to be considered for average price of stock repurchased										30 days				30 days				30 days
Robert G Ruhlman [Member]
Related Party Transaction [Line Items]
Purchase of shares | shares													7,000
Shares price per share													$ 71.07
Number of trading days to be considered for average price of stock repurchased													30 days
X Information Technology [Member]
Related Party Transaction [Line Items]
Service expense | $																			200,000	200,000	$ 200,000
Flex Work Sp [Member]
Related Party Transaction [Line Items]
Labor expense | $																			$ 0	$ 0	$ 400,000
Ownership percentage owned by related party																			50.00%
Baker & Hostetler LLP [Member]
Related Party Transaction [Line Items]
Legal fees | $																			$ 100,000