Loans - Schedule of Loan Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 18,331,580	$ 17,236,103
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,741,425	2,638,195
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	10,347,882	9,544,613
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	9,496,777	8,719,667
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	851,105	824,946
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,859,035	2,867,918
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,383,238	2,185,377
Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	446,280	469,009
Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,208,902	1,139,227
Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	728,056	577,141
Non-PCI Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	16,944,365	15,464,601
Non-PCI Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,549,065	2,357,018
Non-PCI Loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	9,371,815	8,338,594
Non-PCI Loans | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	8,561,851	7,628,328
Non-PCI Loans | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	809,964	710,266
Non-PCI Loans | Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,717,744	2,684,195
Non-PCI Loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,305,741	2,084,794
Non-PCI Loans | Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	373,631	376,213
Non-PCI Loans | Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,208,804	1,139,082
Non-PCI Loans | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	723,306	569,499
PCI Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,387,215	1,771,502
Covered loans	38,700	70,400
PCI Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	192,360	281,177
PCI Loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	976,067	1,206,019
PCI Loans | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	934,926	1,091,339
PCI Loans | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	41,141	114,680
PCI Loans | Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	141,291	183,723
PCI Loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	77,497	100,583
PCI Loans | Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	72,649	92,796
PCI Loans | Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	98	145
PCI Loans | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 4,750	$ 7,642