Loans - Non-Covered and Covered PCI Loans and Non-PCI Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable
Loans	$ 24,111,290	$ 18,331,580
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Loans	4,015,280	2,741,425
Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	13,667,490	10,347,882
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	12,251,231	9,496,777
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Loans	1,416,259	851,105
Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Loans	3,782,972	2,859,035
Consumer
Accounts, Notes, Loans and Financing Receivable
Loans	2,645,548	2,383,238
Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable
Loans	521,797	446,280
Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable
Loans	1,288,902	1,208,902
Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable
Loans	834,849	728,056
Non-PCI Loans
Accounts, Notes, Loans and Financing Receivable
Loans	19,681,255	16,944,365
Non-PCI Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Loans	3,243,622	2,549,065
Non-PCI Loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	10,629,611	9,371,815
Non-PCI Loans | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	9,607,279	8,561,851
Non-PCI Loans | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Loans	1,022,332	809,964
Non-PCI Loans | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Loans	3,331,985	2,717,744
Non-PCI Loans | Consumer
Accounts, Notes, Loans and Financing Receivable
Loans	2,476,037	2,305,741
Non-PCI Loans | Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable
Loans	366,941	373,631
Non-PCI Loans | Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable
Loans	1,288,500	1,208,804
Non-PCI Loans | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable
Loans	820,596	723,306
PCI Loans
Accounts, Notes, Loans and Financing Receivable
Loans	4,430,035	1,387,215
Covered loans	29,100	38,700
PCI Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Loans	771,658	192,360
PCI Loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	3,037,879	976,067
PCI Loans | Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Loans	2,643,952	934,926
PCI Loans | Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable
Loans	393,927	41,141
PCI Loans | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Loans	450,987	141,291
PCI Loans | Consumer
Accounts, Notes, Loans and Financing Receivable
Loans	169,511	77,497
PCI Loans | Consumer | Home equity
Accounts, Notes, Loans and Financing Receivable
Loans	154,856	72,649
PCI Loans | Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable
Loans	402	98
PCI Loans | Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable
Loans	$ 14,253	$ 4,750