Fair Value - Quantitative Information about Significant Unobservable Inputs Used in Fair Value Measurements (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 5,447,271	$ 5,385,091
Percentage of adjusted specific issuer evaluation defaults	100.00%
Percentage of adjusted specific issuer evaluation recoveries	100.00%
Pooled trust preferred securities | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of adjusted specific issuer evaluation defaults	0.00%
Percentage of adjusted specific issuer evaluation recoveries	0.00%
Pooled trust preferred securities | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of adjusted specific issuer evaluation defaults	50.00%
Percentage of adjusted specific issuer evaluation recoveries	25.00%
Significant Unobservable Inputs (Level 3) | Pooled trust preferred securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 7,185	$ 8,222
Significant Unobservable Inputs (Level 3) | Pooled trust preferred securities | Constant Prepayment Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.0000	0.0000
Significant Unobservable Inputs (Level 3) | Pooled trust preferred securities | Minimum | Additional Asset Defaults
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.060	0.062
Significant Unobservable Inputs (Level 3) | Pooled trust preferred securities | Minimum | Expected Asset Recoveries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.000	0.000
Significant Unobservable Inputs (Level 3) | Pooled trust preferred securities | Maximum | Additional Asset Defaults
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.089	0.080
Significant Unobservable Inputs (Level 3) | Pooled trust preferred securities | Maximum | Expected Asset Recoveries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.187	0.191
Significant Unobservable Inputs (Level 3) | Pooled trust preferred securities | Weighted Average | Additional Asset Defaults
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.069	0.068
Significant Unobservable Inputs (Level 3) | Pooled trust preferred securities | Weighted Average | Expected Asset Recoveries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.059	0.060
Significant Unobservable Inputs (Level 3) | States and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		$ 40
Significant Unobservable Inputs (Level 3) | Collateral Dependent Impaired Loans | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 13,676	10,361
Significant Unobservable Inputs (Level 3) | Collateral Dependent Impaired Loans | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 21,880	$ 11,610
Significant Unobservable Inputs (Level 3) | Collateral Dependent Impaired Loans | Discount for Type of Property, Age of Appraisal, and Current Status | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input		0.45
Significant Unobservable Inputs (Level 3) | Collateral Dependent Impaired Loans | Minimum | Discount for Type of Property, Age of Appraisal, and Current Status | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0	0
Significant Unobservable Inputs (Level 3) | Collateral Dependent Impaired Loans | Minimum | Discount for Type of Property, Age of Appraisal, and Current Status | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.25
Significant Unobservable Inputs (Level 3) | Collateral Dependent Impaired Loans | Maximum | Discount for Type of Property, Age of Appraisal, and Current Status | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.80	0.50
Significant Unobservable Inputs (Level 3) | Collateral Dependent Impaired Loans | Maximum | Discount for Type of Property, Age of Appraisal, and Current Status | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.35
Significant Unobservable Inputs (Level 3) | Collateral Dependent Impaired Loans | Weighted Average | Discount for Type of Property, Age of Appraisal, and Current Status | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.48	0.13
Significant Unobservable Inputs (Level 3) | Collateral Dependent Impaired Loans | Weighted Average | Discount for Type of Property, Age of Appraisal, and Current Status | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.30
Significant Unobservable Inputs (Level 3) | Foreclosed Assets | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		$ 21
Significant Unobservable Inputs (Level 3) | Foreclosed Assets | Residential real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 47	$ 22
Significant Unobservable Inputs (Level 3) | Foreclosed Assets | Discount for Type of Property, Age of Appraisal, and Current Status | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input		0.43
Significant Unobservable Inputs (Level 3) | Foreclosed Assets | Discount for Type of Property, Age of Appraisal, and Current Status | Residential real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input		0.21
Significant Unobservable Inputs (Level 3) | Foreclosed Assets | Minimum | Discount for Type of Property, Age of Appraisal, and Current Status | Residential real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.47
Significant Unobservable Inputs (Level 3) | Foreclosed Assets | Maximum | Discount for Type of Property, Age of Appraisal, and Current Status | Residential real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.98
Significant Unobservable Inputs (Level 3) | Foreclosed Assets | Weighted Average | Discount for Type of Property, Age of Appraisal, and Current Status | Residential real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.73