Loans and Allowance for Credit Losses for Loans - Loan Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 32,686,416	$ 32,217,112
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	7,148,644	6,861,708
Loans, deferred income, PPP	57,200	43,200
Commercial and industrial | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,784,017	4,709,569
Commercial and industrial | Commercial and industrial PPP
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,364,627	2,152,139
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	18,709,958	18,470,823
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	16,923,627	16,724,998
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,786,331	1,745,825
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,060,492	4,183,743
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,767,322	2,700,838
Consumer loans | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	409,576	431,553
Consumer loans | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,444,883	1,355,955
Consumer loans | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 912,863	$ 913,330