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Stock-Based Compensation Plans - Summarizes Transactions of SEUs (Detail) - Stock Equivalent Units [Member]
 12 Months Ended
Dec. 31, 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 4.28
Forfeited, Weighted Average Exercise Price 0.00
Outstanding, Weighted Average Exercise Price, Ending balance $ 3.10
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 (48,277)
Forfeited, Number of SEUs | shares (16,975)
Outstanding, Number of SEUs, Ending balance | shares 400,342
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 30.65
Forfeited, Weighted Average Grant-Date Fair Value 34.81
Outstanding, Weighted Average Grant-Date Fair Value $ 46.65