Share-based Compensation - Summary of SARs Activity (Detail)	9 Months Ended
	Sep. 30, 2018 $ / shares shares
Stock-Settled SARs Activity [Member]
Shares
Outstanding, beginning balance (in shares) | shares	1,494,458
Exercised (in shares) | shares	(540,233)
Expired or forfeited (in shares) | shares	(16,204)
Outstanding, ending balance (in shares) | shares	938,021
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 30.85
Exercised (in dollars per share) | $ / shares	26.64
Expired or forfeited (in dollars per share) | $ / shares	36.38
Outstanding, ending balance (in dollars per share) | $ / shares	$ 33.17
Cash Settled S A Rs Activity [Member]
Shares
Outstanding, beginning balance (in shares) | shares	189,568
Exercised (in shares) | shares	(87,665)
Expired or forfeited (in shares) | shares	(2,568)
Outstanding, ending balance (in shares) | shares	99,335
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 28.95
Exercised (in dollars per share) | $ / shares	27.88
Expired or forfeited (in dollars per share) | $ / shares	21.23
Outstanding, ending balance (in dollars per share) | $ / shares	$ 30.10