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Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Investment Securities [Line Items]    
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis $ 5,099  
Amortized Cost 829,188 $ 740,846
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 4,474 4,854
Gross Unrealized Losses (20,785) (14,342)
Securities available for sale, at fair value 812,877 731,358
Due within one year, Estimated Fair Value 5,073  
Due after one but within five years, Amortized Cost 15,727  
Due after one but within five years, Estimated Fair Value 15,541  
Due after five but within ten years, Amortized Cost 26,337  
Due after five but within ten years, Estimated Fair Value 26,409  
Due after ten years, Amortized Cost 15,523  
Due after ten years, Estimated Fair Value 16,594  
Due within one year, Amortized Cost 62,686  
Available-for-sale Securities, Debt Maturities, Single Maturity Date 63,617  
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis [1] 764,832  
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value [1] 747,590  
US Government Agencies Residential Mortgage-backed Securities [Member]    
Investment Securities [Line Items]    
Amortized Cost 83,500  
Securities available for sale, at fair value 82,800  
Residential Mortgage Backed Securities [Member]    
Investment Securities [Line Items]    
Securities available for sale, at fair value 664,788 625,555
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]    
Investment Securities [Line Items]    
Securities available for sale, at fair value 1,098 1,097
Obligations of States and Political Subdivisions [Member]    
Investment Securities [Line Items]    
Securities available for sale, at fair value 27,161 27,410
Corporate Debt Securities [Member]    
Investment Securities [Line Items]    
Securities available for sale, at fair value 21,226 16,493
Pooled Trust Preferred Collateralized Debt Obligations [Member]    
Investment Securities [Line Items]    
Securities available for sale, at fair value 14,132 23,646
Debt Securities [Member]    
Investment Securities [Line Items]    
Amortized Cost 827,518  
Securities available for sale, at fair value 811,207  
US Government Agency Debt Securities Commercial [Domain]    
Investment Securities [Line Items]    
Amortized Cost 73,389 24,611
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 0 0
Gross Unrealized Losses (1,252) (462)
Securities available for sale, at fair value 72,137 24,149
Equity Securities [Member]    
Investment Securities [Line Items]    
Amortized Cost 1,670 1,670
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 0 0
Gross Unrealized Losses 0 0
Securities available for sale, at fair value 1,670 1,670
Debt Securities [Member]    
Investment Securities [Line Items]    
Amortized Cost 827,518 739,176
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 4,474 4,854
Gross Unrealized Losses (20,785) (14,342)
Securities available for sale, at fair value 811,207 729,688
Pooled Trust Preferred Collateralized Debt Obligations [Member]    
Investment Securities [Line Items]    
Amortized Cost 13,602 27,499
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 1,252 526
Gross Unrealized Losses (722) (4,379)
Securities available for sale, at fair value 14,132 23,646
Corporate Debt Securities [Member]    
Investment Securities [Line Items]    
Amortized Cost 20,898 15,907
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 542 590
Gross Unrealized Losses (214) (4)
Securities available for sale, at fair value 21,226 16,493
Obligations of States and Political Subdivisions [Member]    
Investment Securities [Line Items]    
Amortized Cost 27,086 27,083
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 121 327
Gross Unrealized Losses (46) 0
Securities available for sale, at fair value 27,161 27,410
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]    
Investment Securities [Line Items]    
Amortized Cost 1,100 1,098
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 0 0
Gross Unrealized Losses (2) (1)
Securities available for sale, at fair value 1,098 1,097
Residential Mortgage Backed Securities [Member]    
Investment Securities [Line Items]    
Amortized Cost 681,343 632,422
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 1,913 2,622
Gross Unrealized Losses (18,468) (9,489)
Securities available for sale, at fair value 664,788 625,555
US Government Agencies Residential Mortgage-backed Securities [Member]    
Investment Securities [Line Items]    
Amortized Cost 10,100 10,556
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 646 789
Gross Unrealized Losses (81) (7)
Securities available for sale, at fair value $ 10,665 $ 11,338
[1] Mortgage-Backed Securities include an amortized cost of $83.5 million and a fair value of $82.8 million for Obligations of U.S.