Share-Based Compensation - Summary of SAR's Outstanding (Detail) - SARs - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Non-qualified Options
Shares, Outstanding at beginning of year	1,740,786	1,397,611	1,292,142
Shares Granted	402,029	627,971	332,159
Shares, Exercised	(250,326)	(160,946)	(179,702)
Shares, Forfeited	(66,550)	(115,215)	(45,441)
Shares, Expired	(7,948)	(8,635)	(1,547)
Shares, Outstanding at end of year	1,817,991	1,740,786	1,397,611
Shares, Exercisable at end of year	874,942	828,754	736,075
Weighted- Average Exercise Price
Outstanding at beginning of year (in USD per share)	$ 56.15	$ 56.78	$ 42.20
Granted (in USD per share)	98.87	52.13	107.31
Exercised (in USD per share)	48.66	35.37	40.71
Forfeited (in USD per share)	75.38	65.74	75.26
Expired (in USD per share)	108.20	88.65	47.11
Outstanding at end of year (in USD per share)	65.73	56.15	56.78
Exercisable at end of year (in USD per share)	$ 50.86	$ 45.14	$ 35.90