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Securities Available for Sale - Securities Available for Sale (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 1,042,285 $ 1,041,247
Gross unrealized gains 5,643 7,558
Gross unrealized losses (10,116) (8,419)
Securities available for sale, at fair value 1,037,812 1,040,386
US Treasury obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost   7,995
Gross unrealized gains   13
Gross unrealized losses   0
Securities available for sale, at fair value   8,008
Agency Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 19,014 57,123
Gross unrealized gains 0 90
Gross unrealized losses (9) (25)
Securities available for sale, at fair value 19,005 57,188
Mortgage-backed securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 993,548 952,992
Gross unrealized gains 4,914 7,249
Gross unrealized losses (10,095) (8,380)
Securities available for sale, at fair value 988,367 951,861
State and municipal obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 3,259 3,727
Gross unrealized gains 129 19
Gross unrealized losses 0 (3)
Securities available for sale, at fair value 3,388 3,743
Corporate obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 26,047 19,013
Gross unrealized gains 359 35
Gross unrealized losses (12) (11)
Securities available for sale, at fair value 26,394 19,037
Equity Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 417 397
Gross unrealized gains 241 152
Gross unrealized losses 0 0
Securities available for sale, at fair value $ 658 $ 549