Loans - Schedule of Loan Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 25,035,469	$ 18,331,580
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,331,032	2,741,425
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	13,895,407	10,347,882
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	12,407,275	9,496,777
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,488,132	851,105
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,111,400	2,859,035
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,697,630	2,383,238
Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	517,089	446,280
Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,319,571	1,208,902
Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	860,970	728,056
Non-PCI Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	20,845,383	16,944,365
Non-PCI Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	3,590,375	2,549,065
Non-PCI Loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	11,034,657	9,371,815
Non-PCI Loans | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	9,912,309	8,561,851
Non-PCI Loans | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,122,348	809,964
Non-PCI Loans | Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	3,682,984	2,717,744
Non-PCI Loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,537,367	2,305,741
Non-PCI Loans | Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	371,340	373,631
Non-PCI Loans | Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,319,206	1,208,804
Non-PCI Loans | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	846,821	723,306
PCI Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,190,086	1,387,215
Covered loans	27,600	38,700
PCI Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	740,657	192,360
PCI Loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,860,750	976,067
PCI Loans | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,494,966	934,926
PCI Loans | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	365,784	41,141
PCI Loans | Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	428,416	141,291
PCI Loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	160,263	77,497
PCI Loans | Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	145,749	72,649
PCI Loans | Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	365	98
PCI Loans | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 14,149	$ 4,750