Loans - Risk Category of Loans (Detail) - Non-PCI Loans - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	$ 11,723,170	$ 10,695,612
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	2,504,655	2,357,018
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	8,359,833	7,628,328
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	858,682	710,266
Pass
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	11,435,459	10,440,996
Pass | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	2,337,467	2,246,457
Pass | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	8,241,629	7,486,469
Pass | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	856,363	708,070
Special Mention
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	100,885	102,107
Special Mention | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	58,376	44,316
Special Mention | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	42,145	57,591
Special Mention | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	364	200
Substandard
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	180,663	150,913
Substandard | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	102,649	64,649
Substandard | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	76,059	84,268
Substandard | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	1,955	1,996
Doubtful
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	6,163	1,596
Doubtful | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	6,163	1,596
Doubtful | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	0	0
Doubtful | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	$ 0	$ 0