Share-based Compensation - Summary of SARs Activity (Detail)	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Stock-Settled SARs Activity [Member]
Shares
Outstanding at January 1, 2018 (in shares) | shares	1,494,458
Exercised (in shares) | shares	(464,807)
Expired or forfeited (in shares) | shares	(14,947)
Outstanding at June 30, 2018 (in shares) | shares	1,014,704
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 30.85
Exercised (in dollars per share) | $ / shares	27.07
Expired or forfeited (in dollars per share) | $ / shares	36.25
Outstanding, ending balance (in dollars per share) | $ / shares	$ 32.50
Cash Settled S A Rs Activity [Member]
Shares
Outstanding at January 1, 2018 (in shares) | shares	189,568
Exercised (in shares) | shares	(68,955)
Expired or forfeited (in shares) | shares	(2,568)
Outstanding at June 30, 2018 (in shares) | shares	118,045
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 28.95
Exercised (in dollars per share) | $ / shares	28.56
Expired or forfeited (in dollars per share) | $ / shares	21.23
Outstanding, ending balance (in dollars per share) | $ / shares	$ 29.35