Shareholders' Equity and Equity Compensation (Summary of Activity Under Stock Option Plans) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Options Outstanding
Options outstanding at beginning of period (in shares)	116,575	212,038	393,347
Options exercised (in shares)	(83,292)	(95,113)	(178,759)
Options forfeited (in shares)	0	(350)	(2,550)
Options outstanding at end of period (in shares)	33,283	116,575	212,038
Options exercisable at end of period (in shares)	33,283	116,575	204,394
Options vested at the end of the period (in shares)	33,283
Weighted- Average Exercise Price Per Share
Weighted Average Exercise Price Per Share, Outstanding at period start (in dollars per share)	$ 37.76	$ 40.47	$ 40.72
Weighted Average Exercise Price Per Share, Options exercised (in dollars per share)	37.04	43.56	40.90
Weighted Average Exercise Price Per Share, Options forfeited (in dollars per share)	0.00	44.32	40.09
Weighted Average Exercise Price Per Share, Outstanding at period end (in dollars per share)	36.40	$ 37.76	$ 40.47
Weighted Average Exercise Price Per Share, Options exercisable at December 31, 2017	36.40
Weighted Average Exercise Price Per Share, Options vested at December 31, 2017 or expected to vest	$ 36.40
Weighted-Average Remaining Contractual Life in Years
Weighted-Average Remaining Contractual Life in Years, Options outstanding	2 years 2 months 12 days	3 years 2 months 12 days
Weighted-Average Remaining Contractual Life in Years, Options exercisable at December 31, 2017	2 years 2 months 12 days
Weighted-Average Remaining Contractual Life in Years, Options vested at December 31, 2017 or expected to vest	2 years 2 months 12 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Options outstanding	$ 4,177,655	$ 4,552,580
Aggregate Intrinsic Value, Options exercisable at December 31, 2017	4,177,655
Aggregate Intrinsic Value, Options vested at December 31, 2017 or expected to vest	$ 4,177,655