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Employee Benefit Plans Summary of all Directors' Restricted Stock Activity (Detail) - Non Employee Director Restricted Equity Awards - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation cost related to unvested equity awards $ 0.8 $ 0.9 $ 0.7
Number of Shares      
Beginning of year 41,706 53,284 55,839
Add (deduct):      
Granted 38,451 41,706 53,284
Vested (41,706) (53,284) (55,839)
End of year 38,451 41,706 53,284
Weighted Average Grant Date Fair Value      
Beginning of year $ 62.23 $ 40.54 $ 33.24
Granted 73.43 62.23 40.54
Vested 62.23 40.54 33.24
End of year $ 73.43 $ 62.23 $ 40.54