Options Before Adjustment to Option Price to Give Effect to Dilutive Equivalence (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding options, beginning balance	36,000	40,000	43,000
Number of shares outstanding options, exercised	(13,000)	(4,000)	(3,000)
Number of shares outstanding options, ending balance	23,000	36,000	40,000
Exercise weighted average price, outstanding option beginning balance	$ 35.94	$ 35.65	$ 35.22
Exercise weighted average price, outstanding option Exercised	34.90	33.07	29.44
Exercise weighted average price, outstanding option Ending balance	36.53	35.94	35.65
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant price, Beginning balance	34.90	36.53	36.53
Grant price, Exercised	34.90	30.06	29.44
Grant price, ending Balance		34.90	36.53
Grant price, vested and exercisable	36.53
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant price, Beginning balance	36.53	30.06	29.44
Grant price, Exercised	34.90	34.07	29.44
Grant price, ending Balance		$ 36.53	$ 30.06
Grant price, vested and exercisable	$ 36.53