Loans - Recorded Investment in Each Class of Total Loans by Credit Quality Indicator (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 26,309	$ 25,789
Loans, allowance for losses	74	67	$ 54	$ 53
Commercial and Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	24,543	24,915
Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,766	874
Investment grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	21,267	20,473
Investment grade | Commercial and Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	19,501	19,599
Investment grade | Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,766	874
Speculative
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	5,008	5,308
Speculative | Commercial and Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	5,008	5,308
Speculative | Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	0	0
Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	25
Special mention | Commercial and Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	25
Special mention | Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	0
Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	9	8
Substandard | Commercial and Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	9	8
Substandard | Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	0	0
Individually evaluated for impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	25	8
Individually evaluated for impairment | Commercial and Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	25	8
Loans, allowance for losses	1
Individually evaluated for impairment | Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	0	0
Collectively evaluated for impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	26,284	25,781
Collectively evaluated for impairment | Commercial and Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	24,518	24,907
Collectively evaluated for impairment | Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 1,766	$ 874