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Activity in Management Stock Purchase Program (Detail) (Management Stock Purchase Plan, USD $)
12 Months Ended
Dec. 31, 2012
Management Stock Purchase Plan
 
Share Based Compensation [Line Items]  
Number of shares outstanding, beginning balance 217,952
Number of shares, purchased 59,471
Number of shares, vested (49,523)
Number of shares, forfeited (15,733)
Number of shares outstanding, ending balance 212,167
Weighted average grant date fair value outstanding, beginning balance $ 30.38
Weighted average grant date fair value, purchased $ 32.20
Weighted average grant date fair value, vested $ 31.15
Weighted average grant date fair value, forfeited $ 29.80
Weighted average grant date fair value outstanding, ending balance $ 30.76