Loans - Risk Category of Loans (Detail) - Non-PCI Loans - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	$ 9,115,475	$ 8,833,775
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	2,176,942	2,156,549
Commercial real estate loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	6,304,974	6,069,532
Commercial real estate loans | Construction
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	633,559	607,694
Pass
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	8,833,868	8,539,636
Pass | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	2,075,347	2,049,752
Pass | Commercial real estate loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	6,136,061	5,893,354
Pass | Commercial real estate loans | Construction
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	622,460	596,530
Special Mention
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	130,037	148,624
Special Mention | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	57,283	68,243
Special Mention | Commercial real estate loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	71,897	79,279
Special Mention | Commercial real estate loans | Construction
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	857	1,102
Substandard
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	151,570	143,215
Substandard | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	44,312	36,254
Substandard | Commercial real estate loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	97,016	96,899
Substandard | Commercial real estate loans | Construction
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	10,242	10,062
Doubtful
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	0	2,300
Doubtful | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	0	2,300
Doubtful | Commercial real estate loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	0	0
Doubtful | Commercial real estate loans | Construction
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	$ 0	$ 0