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Activity in Management Stock Purchase Program (Detail) - Management Stock Purchase Plan
12 Months Ended
Dec. 31, 2016
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Number of shares outstanding, beginning balance | shares 197,997
Number of shares, purchased | shares 54,359
Number of shares, vested | shares (44,562)
Number of shares, forfeited | shares (7,790)
Number of shares outstanding, ending balance | shares 200,004
Weighted average grant date fair value outstanding, beginning balance | $ / shares $ 39.70
Weighted average grant date fair value, purchased | $ / shares 60.54
Weighted average grant date fair value, vested | $ / shares 32.44
Weighted average grant date fair value, forfeited | $ / shares 47.96
Weighted average grant date fair value outstanding, ending balance | $ / shares $ 46.66