Loans - Risk Category of Loans (Detail) - Non-PCI Loans - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	$ 11,920,880	$ 10,695,612
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	2,549,065	2,357,018
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	8,561,851	7,628,328
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	809,964	710,266
Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	11,631,645	10,440,996
Pass | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	2,375,689	2,246,457
Pass | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	8,447,865	7,486,469
Pass | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	808,091	708,070
Special Mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	110,440	102,107
Special Mention | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	62,071	44,316
Special Mention | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	48,009	57,591
Special Mention | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	360	200
Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	164,045	150,913
Substandard | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	96,555	64,649
Substandard | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	65,977	84,268
Substandard | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	1,513	1,996
Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	14,750	1,596
Doubtful | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	14,750	1,596
Doubtful | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	0	0
Doubtful | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	$ 0	$ 0