Options Before Adjustment to Option Price to Give Effect to Dilutive Equivalence (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Outstanding Options, Beginning balance	23,000	36,000	40,000
Number of Shares, Outstanding Options, Exercised	(23,000)	(13,000)	(4,000)
Number of Shares, Outstanding Options, Ending balance	0	23,000	36,000
Exercise Weighted-Average Price, Outstanding Options, Beginning balance	$ 36.53	$ 35.94	$ 35.65
Exercise Weighted-Average Price, Outstanding Options, Exercised	36.53	34.90	33.07
Exercise Weighted-Average Price, Outstanding Options, Ending balance		36.53	35.94
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant Price, Outstanding Options, Beginning balance	36.53	34.90	36.53
Grant Price, Outstanding Options, Exercised	36.53	34.90	30.06
Grant Price, Outstanding Options, Ending balance		36.53	34.90
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant Price, Outstanding Options, Beginning balance	36.53	36.53	30.06
Grant Price, Outstanding Options, Exercised	$ 36.53	34.90	34.07
Grant Price, Outstanding Options, Ending balance		$ 36.53	$ 36.53