Loans and Allowance for Credit Losses for Loans - Loan Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 32,457,454	$ 32,217,112
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	6,084,455	6,861,708
Loans, deferred income, PPP	40,900	43,200
Commercial and industrial | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,733,771	4,709,569
Commercial and industrial | Commercial and industrial PPP loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,350,684	2,152,139
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	19,264,980	18,470,823
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	17,512,142	16,724,998
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,752,838	1,745,825
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,226,975	4,183,743
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,881,044	2,700,838
Consumer loans | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	410,856	431,553
Consumer loans | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,531,262	1,355,955
Consumer loans | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 938,926	$ 913,330