Stock Option Plan - Summary of Option Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Shares
Outstanding at January 1 (in shares)	570,840	674,585	532,881
Granted (in shares)	237,500	277,500	195,000
Exercised (in shares)	(49,715)	(149,995)	(40,796)
Expired/Forfeited (in shares)	(5,000)	(231,250)	(12,500)
Outstanding December 31 (in shares)	753,625	570,840	674,585
Exercisable at December 31 (in shares)	413,000	377,715	512,085
Weighted Average Exercise Price
Outstanding at January 1 (in usd per share)	$ 41.04	$ 40.40	$ 39.12
Granted (in usd per share)	$ 44.42	$ 39.29	$ 41.82
Exercised (in usd per share)	$ 33.15	$ 31.03	$ 29.03
Expired/Forfeited (in usd per share)	$ 52.16	$ 43.56	$ 45.05
Outstanding December 31 (in usd per share)	$ 42.55	$ 41.04	$ 40.40
Exercisable at December 31 (in usd per share)	$ 42.42	$ 41.41	$ 39.96