Shareholders' Equity and Equity Compensation (Summary of Activity Under Stock Option Plans) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Options Outstanding
Options outstanding at beginning of period (in shares)	212,038	393,347	893,139
Options exercised (in shares)	(95,113)	(178,759)	(476,793)
Options canceled (in shares)	(350)	(2,550)	(22,999)
Options outstanding at end of period (in shares)	116,575	212,038	393,347
Options exercisable at end of period (in shares)	116,575	204,394	364,770
Options vested at the end of the period (in shares)	116,575
Weighted- Average Exercise Price Per Share
Weighted Average Exercise Price Per Share, Outstanding at period start (in dollars per share)	$ 40.47	$ 40.72	$ 43.23
Weighted Average Exercise Price Per Share, Options exercised (in dollars per share)	43.56	40.90	44.60
Weighted Average Exercise Price Per Share, Options canceled (in dollars per share)	44.32	40.09	57.07
Weighted Average Exercise Price Per Share, Outstanding at period end (in dollars per share)	37.76	$ 40.47	$ 40.72
Weighted Average Exercise Price Per Share, Options exercisable at December 31, 2016	37.76
Weighted Average Exercise Price Per Share, Options vested at December 31, 2016 or expected to vest	$ 37.76
Weighted-Average Remaining Contractual Life in Years
Weighted-Average Remaining Contractual Life in Years, Options outstanding	3 years 2 months	3 years 2 months
Weighted-Average Remaining Contractual Life in Years, Options exercisable at December 31, 2016	3 years 2 months
Weighted-Average Remaining Contractual Life in Years, Options vested at December 31, 2016 or expected to vest	3 years 2 months
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Options outstanding	$ 4,552,580	$ 2,557,193
Aggregate Intrinsic Value, Options exercisable at December 31, 2016	4,552,580
Aggregate Intrinsic Value, Options vested at December 31, 2016 or expected to vest	$ 4,552,580