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Stock Option Activity (Detail)
 12 Months Ended
Dec. 31, 2017
$ / shares
shares
Shares  
Outstanding at beginning of year | shares 6,687,150
Exercised | shares (50,000)
Expired | shares (600,000)
Outstanding at end of year | shares 6,037,150
Exercisable at end of year | shares 5,515,968
Weighted-average exercise price  
Outstanding at beginning of year | $ / shares $ 20.68
Exercised | $ / shares 18.63
Expired | $ / shares 24.17
Outstanding at end of year | $ / shares 20.35
Exercisable at end of year | $ / shares $ 20.49