Bank Loans and Related Allowance for Loan Losses (Impaired Assets) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	$ 2	$ 4
Total impaired assets	25	28
Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total nonperforming assets	23	24
Nonperforming Financial Instruments [Member] | Nonaccrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total nonperforming assets	21	21
Nonperforming Financial Instruments [Member] | Other Real Estate Owned [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total nonperforming assets	$ 2	$ 3