Share-Based Compensation - Activity Related to Stock Options (Detail) - $ / shares	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Number of shares
Beginning balance (in shares)	867,682	1,072,829
Granted (in shares)	81,313	77,098
Exercised (in shares)	(95,265)	(234,363)
Forfeited or expired (in shares)	(15,733)	(13,446)
Ending balance (in shares)	837,997	902,118
Exercisable options (in shares)	570,148	581,423
Weighted Average Exercise Price
Beginning balance (in usd per share)	$ 55.92	$ 53.47
Granted (in usd per share)	62.53	65.30
Exercised (in usd per share)	50.64	48.36
Forfeited or expired (in usd per share)	66.57	60.82
Ending balance (in usd per share)	56.96	55.70
Exercisable options (in usd per share)	$ 56.53	$ 55.65