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Stock-Based Compensation (Detail) - (Table 6) - USD ($)
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]    
Non-vested at January 1 3,916,000 4,191,000
Non-vested at January 1 (in Dollars per share) $ 26.05 $ 25.93
Granted 160,000 166,000
Granted (in Dollars per share) $ 45.37 $ 36.61
Vested (120,000) (125,000)
Vested (in Dollars per share) $ 34.52 $ 33.15
Forfeited (6,000) (3,000)
Forfeitures (in dollars per share) $ 37.65 $ 34.77
Non-vested at March 31 3,950,000 4,229,000
Non-vested at March 31 (in Dollars per share) $ 26.56 $ 26.13