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Compensation Plans - Roll-Forward of Outstanding Restricted Stock Grants Issued (Detail)
3 Months Ended
Mar. 31, 2021
$ / shares
shares
Shares  
Outstanding shares, Beginning Balance (shares) 183,718
Outstanding shares, Ending Balance (shares) 215,352
Restricted stock  
Shares  
Outstanding shares, Beginning Balance (shares) 183,718
Shares issued (shares) 52,434
Grants vested (shares) (7,600)
Grants forfeited (shares) (13,200)
Outstanding shares, Ending Balance (shares) 215,352
Weighted-Average Grant Date Price per Share  
Outstanding, Beginning of Period (usd per share) | $ / shares $ 173.80
Grants issued (usd per share) | $ / shares 155.64
Grants vested (usd per share) | $ / shares 174.98
Grants forfeited (usd per share) | $ / shares 156.88
Outstanding, End of Period (usd per share) | $ / shares $ 168.34