Loans - Non-Covered and Covered PCI Loans and Non-PCI Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable
Loans	$ 23,234,716	$ 18,331,580
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Loans	3,829,525	2,741,425
Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	13,290,562	10,347,882
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	11,913,830	9,496,777
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Loans	1,376,732	851,105
Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Loans	3,528,682	2,859,035
Consumer
Accounts, Notes, Loans and Financing Receivable
Loans	2,585,947	2,383,238
Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable
Loans	520,849	446,280
Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable
Loans	1,281,735	1,208,902
Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable
Loans	783,363	728,056
Non-PCI Loans
Accounts, Notes, Loans and Financing Receivable
Loans	18,587,015	16,944,365
Non-PCI Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Loans	3,021,524	2,549,065
Non-PCI Loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	10,106,347	9,371,815
Non-PCI Loans | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	9,109,645	8,561,851
Non-PCI Loans | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Loans	996,702	809,964
Non-PCI Loans | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Loans	3,051,429	2,717,744
Non-PCI Loans | Consumer
Accounts, Notes, Loans and Financing Receivable
Loans	2,407,715	2,305,741
Non-PCI Loans | Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable
Loans	358,043	373,631
Non-PCI Loans | Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable
Loans	1,281,303	1,208,804
Non-PCI Loans | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable
Loans	768,369	723,306
PCI Loans
Accounts, Notes, Loans and Financing Receivable
Loans	4,647,701	1,387,215
Covered loans	31,100	38,700
PCI Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Loans	808,001	192,360
PCI Loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	3,184,215	976,067
PCI Loans | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	2,804,185	934,926
PCI Loans | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Loans	380,030	41,141
PCI Loans | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Loans	477,253	141,291
PCI Loans | Consumer
Accounts, Notes, Loans and Financing Receivable
Loans	178,232	77,497
PCI Loans | Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable
Loans	162,806	72,649
PCI Loans | Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable
Loans	432	98
PCI Loans | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable
Loans	$ 14,994	$ 4,750