Stock-Based Compensation Plans - Summarizes Transactions of SEUs (Detail) - Stock Equivalent Units [Member]	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, Number of SEUs, Beginning balance | shares	418,936
Granted at discount, Number of SEUs | shares	119,315
Granted at market value, Number of SEUs | shares	4,359
Exercised, Number of SEUs | shares	(118,772)
Forfeited, Number of SEUs | shares	(750)
Outstanding, Number of SEUs, Ending balance | shares	423,088
Outstanding, Weighted Average Exercise Price, Beginning balance	$ 3.47
Granted at discount, Weighted Average Exercise Price	0.00
Granted at market value, Weighted Average Exercise Price	81.07
Exercised, Weighted Average Exercise Price	2.42
Forfeited, Weighted Average Exercise Price	0.00
Outstanding, Weighted Average Exercise Price, Ending balance	3.59
Outstanding, Weighted Average Grant-Date Fair Value	52.45
Granted at discount, Weighted Average Grant-Date Fair Value	71.70
Granted at market value, Weighted Average Grant-Date Fair Value	22.69
Exercised, Weighted Average Grant-Date Fair Value	47.35
Forfeited, Weighted Average Grant-Date Fair Value	51.05
Outstanding, Weighted Average Grant-Date Fair Value	$ 59.01