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Investment Securities (Disclosure Regarding Length of Time on Securities Available for Sale with Temporary Impairment) (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value $ 446,629 $ 568,290
Less than 12 months, gross unrealized losses (3,968) (8,405)
12 months or longer, fair value 51,872 1,055
12 months or longer, gross unrealized losses (982) (14)
Total, fair value 498,501 569,345
Total, gross unrealized losses (4,950) (8,419)
U.S. Treasury Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value 5,994  
Less than 12 months, gross unrealized losses (1)  
12 months or longer, fair value 0  
12 months or longer, gross unrealized losses 0  
Total, fair value 5,994  
Total, gross unrealized losses (1) 0
Agency Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value 16,008 14,000
Less than 12 months, gross unrealized losses (11) (25)
12 months or longer, fair value 0 0
12 months or longer, gross unrealized losses 0 0
Total, fair value 16,008 14,000
Total, gross unrealized losses (11) (25)
Mortgage-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value 424,627 553,629
Less than 12 months, gross unrealized losses (3,956) (8,377)
12 months or longer, fair value 50,881 65
12 months or longer, gross unrealized losses (972) (3)
Total, fair value 475,508 553,694
Total, gross unrealized losses (4,928) (8,380)
State and municipal obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value   661
Less than 12 months, gross unrealized losses   (3)
12 months or longer, fair value   0
12 months or longer, gross unrealized losses   0
Total, fair value   661
Total, gross unrealized losses   (3)
Corporate Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, fair value 0 0
Less than 12 months, gross unrealized losses 0 0
12 months or longer, fair value 991 990
12 months or longer, gross unrealized losses (10) (11)
Total, fair value 991 990
Total, gross unrealized losses $ (10) $ (11)