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Investment Securities (Narrative) (Detail)
 3 Months Ended 9 Months Ended
Sep. 30, 2017
USD ($)
security
position
Sep. 30, 2016
USD ($)
Sep. 30, 2017
USD ($)
security
position
Sep. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
security
position
Schedule of Held-to-maturity Securities [Line Items]          
Securities available for sale, at fair value $ 1,028,305,000   $ 1,028,305,000   $ 1,040,386,000
Investment securities held to maturity $ 481,845,000   $ 481,845,000   $ 488,183,000
Total number of all held to maturity and available for sale securities in an unrealized loss position | security 253   253   419
Amortized cost $ 1,025,030,000   $ 1,025,030,000   $ 1,041,247,000
Proceeds from the sale of securities available for sale $ 0 $ 1,200,000 $ 0    
Gross gains   0   $ 95,000  
Gross losses   45,000   45,000  
Proceeds from maturities, prepayments and calls of short-term investments       3,400,000  
Proceeds from calls on securities available for sale   0   0  
Number of securities in an unrealized loss position | position 82   82   87
Other-than-temporary impairment charges, net $ 0   $ 0    
Proceeds from calls of held to maturity securities 8,100,000 20,300,000 28,700,000 35,200,000  
Recognized gain on calls of securities, held-to-maturity portfolio 39,000 2,000 50,000 4,000  
Recognized loss on calls of securities held-to-maturity portfolio 3,000 $ 0 3,000 $ 0  
Investment securities held to maturity, fair value $ 490,425,000   $ 490,425,000   $ 489,287,000
Number of securities in an unrealized loss position | security 171   171   332
Private Label Mortgage-Backed Securities [Member]          
Schedule of Held-to-maturity Securities [Line Items]          
Amortized cost $ 50,000   $ 50,000    
Number of securities in an unrealized loss position | position 2   2    
Unrealized losses     $ 2,000    
Mortgage-Backed Securities [Member]          
Schedule of Held-to-maturity Securities [Line Items]          
Securities available for sale, at fair value $ 968,412,000   968,412,000   $ 951,861,000
Investment securities held to maturity 475,000   475,000   893,000
Amortized cost 965,863,000   965,863,000   952,992,000
Held-to-maturity securities, amortized cost 475,000   475,000    
Investment securities held to maturity, fair value 492,000   492,000   924,000
Equity Securities [Member]          
Schedule of Held-to-maturity Securities [Line Items]          
Securities available for sale, at fair value 598,000   598,000   549,000
Amortized cost $ 397,000   $ 397,000   $ 397,000