Loans - Risk Category of Loans (Detail) - Non-PCI Loans - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	$ 14,625,032	$ 11,920,880
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	3,590,375	2,549,065
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	9,912,309	8,561,851
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	1,122,348	809,964
Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	14,349,491	11,631,645
Pass | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	3,399,426	2,375,689
Pass | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	9,828,744	8,447,865
Pass | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	1,121,321	808,091
Special Mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	63,103	110,440
Special Mention | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	31,996	62,071
Special Mention | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	30,892	48,009
Special Mention | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	215	360
Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	144,842	164,045
Substandard | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	92,320	96,555
Substandard | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	51,710	65,977
Substandard | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	812	1,513
Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	67,596	14,750
Doubtful | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	66,633	14,750
Doubtful | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	963	0
Doubtful | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Non-PCI Loans	$ 0	$ 0