Loans and Allowance for Loan Losses - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
PCI Loans		$ 800,000
Current	$ 11,194,144,000	10,455,845,000
Total Past Due	86,369,000	83,738,000
Proceeds from sales of loans held for sale	70,543,000	89,522,000	$ 97,690,000
Non- Accrual Loans	59,142,000	70,259,000
Restructured loans	41,000,000	52,500,000
Commitments to lend to borrowers with loan modifications classified as TDR's	3,100,000	800,000
Default payment of troubled restructuring, commercial real estate loan	0
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	4,502,508,000	4,371,695,000
Total Past Due	50,532,000	39,111,000
Non- Accrual Loans	$ 38,644,000	$ 35,777,000
Number of Contracts	1	3
Pre-Modification Outstanding Recorded Investment	$ 7,200,000	$ 24,800,000
Post-Modification Outstanding Recorded Investment	$ 7,200,000	24,800,000
Real estate - residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of Contracts	1
Pre-Modification Outstanding Recorded Investment	$ 97,000
Post-Modification Outstanding Recorded Investment	$ 98,000
Revolving Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of days past due to consider loan as a loss and charged off	90 days
Closed-End Retail Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of days past due to consider loan as a loss and charged off	120 days
Open-End Retail Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of days past due to consider loan as a loss and charged off	180 days
30-89 Days Past Due [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Past Due	$ 24,136,000	10,114,000
30-89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Past Due	11,216,000	3,285,000
Greater than 90 days Past Due and Accruing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Past Due	3,091,000	3,365,000
Greater than 90 days Past Due and Accruing [Member] | Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Past Due	$ 672,000	49,000
PCI loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
PCI Loans		800,000
Current		713,000
PCI loans [Member] | 30-89 Days Past Due [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Past Due		$ 87,000