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Investment Securities - Gross Unrealized Losses on Debt Securities (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value $ 503,911 $ 289,271
Less than 12 months, Unrealized Loss (16,275) (2,211)
12 months or more, Fair Value 168,560 183,423
12 months or more, Unrealized Loss (8,992) (3,719)
Total Fair Value 672,471 472,694
Total Unrealized Loss (25,267) (5,930)
Less than 12 months, Fair Value, held to maturity 335,850 94,505
Less than 12 months, Unrealized Loss, held to maturity (5,730) (574)
12 months or more, Fair Value, held to maturity 94,337 98,004
12 months or more, Unrealized Loss, held to maturity (3,762) (1,352)
Total Fair Value, held to maturity 430,187 192,509
Total Unrealized Loss, held to maturity (9,492) (1,926)
Obligations of U.S. Government Corporations and Agencies [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 437,859 284,367
Less than 12 months, Unrealized Loss (14,490) (2,176)
12 months or more, Fair Value 152,004 166,338
12 months or more, Unrealized Loss (7,674) (3,425)
Total Fair Value 589,863 450,705
Total Unrealized Loss (22,164) (5,601)
Less than 12 months, Fair Value, held to maturity 326,966 93,017
Less than 12 months, Unrealized Loss, held to maturity (5,578) (567)
12 months or more, Fair Value, held to maturity 91,801 95,367
12 months or more, Unrealized Loss, held to maturity (3,633) (1,322)
Total Fair Value, held to maturity 418,767 188,384
Total Unrealized Loss, held to maturity (9,211) (1,889)
Obligations of States and Political Subdivisions [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 66,052 4,904
Less than 12 months, Unrealized Loss (1,785) (35)
12 months or more, Fair Value 16,556 17,085
12 months or more, Unrealized Loss (1,318) (294)
Total Fair Value 82,608 21,989
Total Unrealized Loss (3,103) (329)
Less than 12 months, Fair Value, held to maturity 8,884 1,488
Less than 12 months, Unrealized Loss, held to maturity (152) (7)
12 months or more, Fair Value, held to maturity 2,536 2,637
12 months or more, Unrealized Loss, held to maturity (129) (30)
Total Fair Value, held to maturity 11,420 4,125
Total Unrealized Loss, held to maturity $ (281) $ (37)