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Employee Savings Plans - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expenses related ESOP $ 8,241 $ 7,849 $ 7,455
Shares vested under ESOP 3,424,000    
Matching percentage of employer to employee contributions 100.00%    
Matching contributions to 401(k) retirement savings plan $ 22,101 15,456 $ 14,178
Temporary equity, shares issued (in shares) 94,501    
Deferred compensation $ 11,717 8,728  
Temporary equity, other changes 7,457    
Temporary equity, accretion to redemption value, adjustment $ 4,260    
Employee Stock Ownership Plan E S O P Plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Maximum number of shares of common stock available for issuance 7,200,000    
Fair value, measurements, recurring      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Assets, fair value disclosure $ 54,244 49,628  
Level 1 | Fair value, measurements, recurring      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Assets, fair value disclosure 54,244 49,330  
Level 1 | Fair value, measurements, recurring | Non-qualified deferred compensation assets      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Assets, fair value disclosure $ 54,244 $ 49,330