Bank Loans and Related Allowance for Loan Losses (Impaired Assets) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	$ 8	$ 11
Total impaired assets	39	42
Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total nonperforming assets	31	31
Nonperforming Financial Instruments [Member] | Nonaccrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total nonperforming assets	29	28
Nonperforming Financial Instruments [Member] | Other Real Estate Owned [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total nonperforming assets	$ 2	$ 3