Stock-Based Compensation Plans - Summarizes Transactions of SEUs (Detail) - Stock Equivalent Units [Member]	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, Weighted Average Exercise Price, Beginning balance	$ 3.77
Granted at discount, Weighted Average Exercise Price	0.00
Granted at market value, Weighted Average Exercise Price	70.60
Exercised, Weighted Average Exercise Price	5.47
Forfeited, Weighted Average Exercise Price	0.00
Outstanding, Weighted Average Exercise Price, Ending balance	$ 3.03
Outstanding, Number of SEUs, Beginning balance | shares	279,815
Granted at discount, Number of SEUs | shares	180,249
Granted at market value, Number of SEUs | shares	5,530
Exercised, Number of SEUs | shares	(34,054)
Forfeited, Number of SEUs | shares	(16,075)
Outstanding, Number of SEUs, Ending balance | shares	415,465
Outstanding, Weighted Average Grant-Date Fair Value	$ 33.40
Granted at discount, Weighted Average Grant-Date Fair Value	62.76
Granted at market value, Weighted Average Grant-Date Fair Value	16.84
Exercised, Weighted Average Grant-Date Fair Value	27.09
Forfeited, Weighted Average Grant-Date Fair Value	33.77
Outstanding, Weighted Average Grant-Date Fair Value	$ 46.41