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Stock-Based Compensation - Performance Share Units and Restricted Stock Units (Detail) - Performance Share Units and Restricted Stock Units [Member] - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock awards, Outstanding, Beginning Balance 321,912 189,042 140,565
Granted, stock units 172,795 211,168 115,896
Vested and common stock issued, stock units (81,561) (64,950) (52,133)
Forfeited, stock units (35,811) (13,348) (15,286)
Stock awards, Outstanding, Ending Balance 377,335 321,912 189,042
Weighted average grant date fair value, Outstanding, Beginning Balance $ 55.16 $ 54.14 $ 56.88
Granted, Weighted Average Grant Date Fair Value 66.03 55.62 48.32
Vested and common stock issued, Weighted Average Grant Date Fair Value 56.32 53.64 48.19
Forfeited, Weighted Average Grant Date Fair Value 55.04 55.39 55.48
Weighted average grant date fair value, Outstanding, Ending Balance $ 59.90 $ 55.16 $ 54.14