Shareholders' Equity and Equity Compensation (Performance Based Restricted Stock Awards) (Detail) - Performance Shares - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Awards Outstanding
Non-vested awards outstanding, beginning balance (in shares)	151,769	107,229	92,437
Awards granted (in shares)	56,147	84,443	51,475
Stock issued (in shares)	(34,442)	(25,397)	(20,910)
Awards forfeited or expired (in shares)	(4,272)	(14,506)	(15,773)
Non-vested awards outstanding, ending balance (in shares)	169,202	151,769	107,229
Weighted- Average Grant Date Fair Value
Weighted-Average Grant Date Fair Value, Outstanding at period beginning (in dollars per share)	$ 89.72	$ 66.13	$ 52.75
Weighted-Average Grant Date Fair Value, Awards granted (in dollars per share)	110.77	69.01	78.01
Weighted-Average Grant Date Fair Value, Stock issued (in dollars per share)	86.59	72.68	41.27
Weighted-Average Grant Date Fair Value, Awards forfeited or expired (in dollars per share)	99.35	104.83	59.45
Weighted-Average Grant Date Fair Value, Outstanding at period end (in dollars per share)	$ 97.16	$ 89.72	$ 66.13