Bank Loans and Related Allowance for Loan Losses (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans accruing interest contractually 90 days or more past due	$ 0	$ 0
Balance	16,504,000,000	15,429,000,000
First Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance	$ 10,016,000,000	9,134,000,000
HELOC's [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Loan Term	30 years
Financing Receivable, Initial Draw Period	10 years
Financing Receivable, Convert to Amortizing Loans, Period	20 years
Balance	$ 1,943,000,000	2,350,000,000
Percent of loan balance outstanding, borrowers paid only minimum interest due	38.00%
Home Equity Secured By Second Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance	$ 1,500,000,000
First Mortgage and Home Equity Loans and Lines of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unamortized premiums and discounts and direct origination costs	77,000,000	$ 78,000,000
Loans pledged as collateral	11,100,000,000
Adjustable Rate First Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount of mortgages	$ 9,000,000,000
Percent of loans with interest-only payments	33.00%
Percent of interest only adjustable rate	58.00%
Adjustable Rate First Mortgage [Member] | Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fixed rate interest rate period	3 years
Interest-only reset period	3 years
Adjustable Rate First Mortgage [Member] | Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fixed rate interest rate period	10 years