Stock-Based Compensation Plans - Summarizes Transactions of SEUs (Detail) - Stock Equivalent Units [Member]	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, Weighted Average Exercise Price, Beginning balance	$ 3.10
Granted at discount, Weighted Average Exercise Price	0.00
Granted at market value, Weighted Average Exercise Price	68.20
Exercised, Weighted Average Exercise Price	3.57
Forfeited, Weighted Average Exercise Price	1.40
Outstanding, Weighted Average Exercise Price, Ending balance	$ 3.23
Outstanding, Number of SEUs, Beginning balance | shares	400,342
Granted at discount, Number of SEUs | shares	101,278
Granted at market value, Number of SEUs | shares	5,658
Exercised, Number of SEUs | shares	(48,852)
Forfeited, Number of SEUs | shares	(16,554)
Outstanding, Number of SEUs, Ending balance | shares	441,872
Outstanding, Weighted Average Grant-Date Fair Value	$ 46.65
Granted at discount, Weighted Average Grant-Date Fair Value	57.44
Granted at market value, Weighted Average Grant-Date Fair Value	16.23
Exercised, Weighted Average Grant-Date Fair Value	28.12
Forfeited, Weighted Average Grant-Date Fair Value	37.14
Outstanding, Weighted Average Grant-Date Fair Value	$ 51.14