Loans and Allowance for Credit Losses for Loans - Loans Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 32,415,586	$ 29,699,208
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	6,903,345	4,825,997
Loans, PPP	2,300,000
Loans, deferred income, PPP	54,400
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	18,536,362	17,643,759
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	16,815,587	15,996,741
Commercial real estate | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,720,775	1,647,018
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	4,284,595	4,377,111
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,691,284	2,852,341
Consumer loans | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	457,083	487,272
Consumer loans | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,341,659	1,451,623
Consumer loans | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 892,542	$ 913,446