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Stock-Based Compensation Plans - Summarizes Transactions of SEUs (Detail) - Stock Equivalent Units [Member]
 12 Months Ended
Dec. 31, 2018
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Outstanding, Number of SEUs, Beginning balance | shares 400,342
Granted at discount, Number of SEUs | shares 115,229
Granted at market value, Number of SEUs | shares 5,658
Exercised, Number of SEUs | shares (78,239)
Forfeited, Number of SEUs | shares (24,054)
Outstanding, Number of SEUs, Ending balance | shares 418,936
Outstanding, Weighted Average Exercise Price, Beginning balance $ 3.10
Granted at discount, Weighted Average Exercise Price 2.36
Granted at market value, Weighted Average Exercise Price 68.20
Exercised, Weighted Average Exercise Price 5.40
Forfeited, Weighted Average Exercise Price 0.96
Outstanding, Weighted Average Exercise Price, Ending balance 3.47
Outstanding, Weighted Average Grant-Date Fair Value 46.65
Granted at discount, Weighted Average Grant-Date Fair Value 57.16
Granted at market value, Weighted Average Grant-Date Fair Value 16.23
Exercised, Weighted Average Grant-Date Fair Value 30.44
Forfeited, Weighted Average Grant-Date Fair Value 41.58
Outstanding, Weighted Average Grant-Date Fair Value $ 52.45