Stock Plans - Summary of Stock Option Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding Beginning balance, Shares	1,695,380	1,784,660	1,818,640
Granted, Shares	138,519	188,407	196,469
Exercised, Shares	(190,743)	(277,687)	(223,022)
Expired, Shares	(1,071)		(5,093)
Forfeited, Shares	(23,524)		(2,334)
Outstanding Ending balance, Shares	1,618,561	1,695,380	1,784,660
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Outstanding Beginning balance, Weighted Average Exercise Price	$ 34.55	$ 33.05	$ 30.94
Granted, Weighted Average Exercise Price	$ 51.69	$ 49.03	$ 37.33
Exercised, Weighted Average Exercise Price	$ 36.31	$ 34.72	$ 18.93
Expired, Weighted Average Exercise Price	$ 45.68		$ 43.33
Forfeited, Weighted Average Exercise Price	$ 38.82		$ 36.69
Outstanding Ending balance,Weighted Average Exercise Price	$ 35.74	$ 34.55	$ 33.05
Exercisable at end of period	1,147,712	1,178,657	1,100,300