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Stock Option Activity (Detail)
 3 Months Ended
Mar. 31, 2018
$ / shares
shares
Shares  
Outstanding at beginning of period | shares 6,037,150
Exercised | shares (40,000)
Outstanding at end of period | shares 5,997,150
Exercisable at end of year | shares 5,610,625
Weighted Average Exercise Price Per Share  
Outstanding at beginning of period | $ / shares $ 20.35
Exercised | $ / shares 12.12
Outstanding at end of period | $ / shares 20.41
Exercisable at end of year | $ / shares $ 20.52