Loans - Risk Category of Loans (Detail) - Non-PCI Loans - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	$ 8,683,941	$ 7,489,763
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	2,152,638	1,959,927
Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	6,009,760	5,053,742
Construction
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	521,543	476,094
Pass
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	8,394,864	7,223,802
Pass | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	2,045,297	1,865,472
Pass | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	5,842,344	4,903,185
Pass | Construction
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	507,223	455,145
Special Mention
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	129,178	92,608
Special Mention | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	66,354	50,453
Special Mention | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	61,842	40,232
Special Mention | Construction
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	982	1,923
Substandard
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	158,064	170,809
Substandard | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	39,152	44,002
Substandard | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	105,574	110,325
Substandard | Construction
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	13,338	16,482
Doubtful
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	1,835	2,544
Doubtful | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	1,835	0
Doubtful | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	0	0
Doubtful | Construction
Accounts, Notes, Loans and Financing Receivable
Total Non-PCI Loans	$ 0	$ 2,544