Shareholders' Equity and Equity Compensation (Deferred Stock Units) (Detail) - Deferred Stock Units - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Awards Outstanding
Non-vested awards outstanding, beginning balance (in shares)	11,900	23,950	30,150
Awards granted (in shares)	9,250	11,900	10,300
Stock issued (in shares)	(11,900)	(23,950)	(16,500)
Non-vested awards outstanding, ending balance (in shares)	9,250	11,900	23,950
Weighted- Average Grant Date Fair Value
Weighted-Average Grant Date Fair Value, Outstanding at period beginning (in dollars per share)	$ 58.82	$ 27.22	$ 24.43
Weighted-Average Grant Date Fair Value, Awards granted (in dollars per share)	109.48	58.82	73.79
Weighted-Average Grant Date Fair Value, Stock issued (in dollars per share)	109.36	52.69	51.20
Weighted-Average Grant Date Fair Value, Outstanding at period end (in dollars per share)	$ 109.48	$ 58.82	$ 27.22