Information Relating to Option Activity (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Long-Term Incentive Compensation Plan (LTIP) [Member]
Number of Shares
Outstanding, Beginning Balance | shares	1,163,849
Granted | shares	151,279
Canceled | shares	(18,291)
Expired | shares	(678)
Exercised | shares	(248,698)
Outstanding, Ending Balance | shares	1,047,461
Exercisable | shares	345,811
Weighted Average Price Per Share
Outstanding, Beginning Balance | $ / shares	$ 47.10
Granted | $ / shares	75.25
Canceled | $ / shares	53.22
Expired | $ / shares	49.97
Exercised | $ / shares	42.57
Outstanding, Ending Balance | $ / shares	52.13
Exercisable | $ / shares	$ 44.89
Weighted Average Remaining Contractual Term
Outstanding	6 years 4 months 24 days
Exercisable	4 years
Aggregate Intrinsic Value
Outstanding | $	$ 20,722,166
Exercisable | $	$ 9,348,718
2002 Plan [Member]
Number of Shares
Outstanding, Beginning Balance | shares	91,461
Expired | shares	(4,304)
Exercised | shares	(55,471)
Outstanding, Ending Balance | shares	31,686
Exercisable | shares	31,686
Weighted Average Price Per Share
Outstanding, Beginning Balance | $ / shares	$ 39.63
Expired | $ / shares	38.65
Exercised | $ / shares	38.96
Outstanding, Ending Balance | $ / shares	40.93
Exercisable | $ / shares	$ 40.93
Weighted Average Remaining Contractual Term
Outstanding	10 months 24 days
Exercisable	10 months 24 days
Aggregate Intrinsic Value
Outstanding | $	$ 981,949
Exercisable | $	$ 981,949