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Employee Benefit Plans Summary of All Employee Restricted Equity Award Activity (Detail) - Employee Restricted Equity Awards - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares      
Beginning of year 6,951,373 7,725,227 9,051,564
Add (deduct):      
Granted 2,383,475 1,870,660 2,489,976
Vested (3,220,671) (2,422,700) (3,682,644)
Forfeited (255,329) (221,814) (133,669)
End of year 5,858,848 6,951,373 7,725,227
Weighted Average Grant Date Fair Value      
Beginning of year $ 26.18 $ 23.37 $ 21.27
Granted 32.01 31.54 25.20
Vested 22.53 21.50 19.53
Forfeited 28.03 24.64 21.63
End of year $ 30.47 $ 26.18 $ 23.37
Unrecognized compensation cost related to unvested equity awards $ 87.1    
Period of recognition of compensation expense related to unvested equity awards 2 years 2 months    
Dividend Equivalent Units      
Add (deduct):      
Granted 157,396 165,045 196,947
Weighted Average Grant Date Fair Value      
Granted $ 0 $ 0 $ 0
Performance Shares      
Weighted Average Grant Date Fair Value      
Target shares 1,513,779    
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Expected To Vest Number 3,244,111