Loans - Asset Quality Indicators (Details)	Mar. 31, 2020 USD ($)		Dec. 31, 2019 USD ($)	[1]	Mar. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 33,378,303,000	[1]	$ 31,061,111,000		$ 27,990,048,000
Commercial | PD Grade 1
Accounts, Notes, Loans and Financing Receivable [Line Items]
Lowest expected default probability	1
Commercial | Loss
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loan grades	16
Commercial | Special Mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loan grades	13
Commercial | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loan grades	14
Commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loan grades	15
Commercial | Minimum | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loan grades	1
Commercial | Minimum | Special Mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loan grades	13
Commercial | Minimum | Loan Reassessed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 1,000,000
Commercial | Minimum | PD Grade 13
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 500,000
Commercial | Minimum | LGD Grade 1
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loan grades	1
Commercial | Maximum | Loss
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loan grades	16
Commercial | Maximum | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loan grades	12
Commercial | Maximum | LGD Grade 12
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loan grades	12

[1] March 31, 2020 and December 31, 2019 include $20.8 million and $18.8 million, respectively, of held-to-maturity consumer mortgage loans secured by residential real estate in process of foreclosure.