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Stock-Based Compensation Plans - Summarizes Transactions of SEUs (Detail) - Stock Equivalent Units [Member]
 6 Months Ended
Jun. 30, 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 121,815
Granted at market value, Number of SEUs | shares 4,359
Exercised, Number of SEUs | shares (129,096)
Forfeited, Number of SEUs | shares (1,500)
Outstanding, Number of SEUs, Ending balance | shares 414,514
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 2.44
Forfeited, Weighted Average Exercise Price 11.57
Outstanding, Weighted Average Exercise Price, Ending balance 3.55
Outstanding, Weighted Average Grant-Date Fair Value 52.45
Granted at discount, Weighted Average Grant-Date Fair Value 71.36
Granted at market value, Weighted Average Grant-Date Fair Value 22.69
Exercised, Weighted Average Grant-Date Fair Value 46.95
Forfeited, Weighted Average Grant-Date Fair Value 48.13
Outstanding, Weighted Average Grant-Date Fair Value $ 59.42