Loans - Risk Category of Loans (Detail) - Non-PCI Loans - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	$ 11,444,355	$ 10,695,612
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	2,417,924	2,357,018
Commercial Real Estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	8,201,235	7,628,328
Commercial Real Estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	825,196	710,266
Pass
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	11,123,678	10,440,996
Pass | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	2,237,178	2,246,457
Pass | Commercial Real Estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	8,063,898	7,486,469
Pass | Commercial Real Estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	822,602	708,070
Special Mention
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	128,743	102,107
Special Mention | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	76,419	44,316
Special Mention | Commercial Real Estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	51,960	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	185,641	150,913
Substandard | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	98,034	64,649
Substandard | Commercial Real Estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	85,377	84,268
Substandard | Commercial Real Estate | Construction
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	2,230	1,996
Doubtful
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	6,293	1,596
Doubtful | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Credit exposure - by internally assigned risk rating	6,293	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