Share-Based Compensation - Summary of SARs Outstanding (Detail)	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Stock Awards
Outstanding at beginning of period (in shares) | shares	15,705
Exercised (in shares) | shares	(9,083)
Expired (in shares) | shares	0
Outstanding at end of period (in shares) | shares	6,622
Exercisable at end of period (in shares) | shares	6,622
Weighted- Average Exercise Price
Outstanding at beginning of period (usd per share) | $ / shares	$ 26.34
Exercised (usd per share) | $ / shares	31.26
Expired (usd per share) | $ / shares	0.00
Outstanding at end of period (usd per share) | $ / shares	19.59
Exercisable at end of period (usd per share) | $ / shares	$ 19.59
SARs | 2015 Plan
Stock Awards
Outstanding at beginning of period (in shares) | shares	1,817,991
Granted (in shares) | shares	87,310
Exercised (in shares) | shares	(384,528)
Forfeited (in shares) | shares	(35,417)
Expired (in shares) | shares	(438)
Outstanding at end of period (in shares) | shares	1,484,918
Exercisable at end of period (in shares) | shares	797,045
Weighted- Average Exercise Price
Outstanding at beginning of period (usd per share) | $ / shares	$ 65.73
Granted (usd per share) | $ / shares	149.57
Exercised (usd per share) | $ / shares	58.18
Forfeited (usd per share) | $ / shares	79.69
Expired (usd per share) | $ / shares	108.20
Outstanding at end of period (usd per share) | $ / shares	72.28
Exercisable at end of period (usd per share) | $ / shares	$ 58.20