Bank Loans and Related Allowance for Loan Losses (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total bank loans	$ 16,590	$ 16,504
First Mortgage and HELOC [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans pledged as collateral	11,100
Adjustable Rate First Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Bank loans	$ 9,300
Percent of loans with interest-only payments	31.00%
Percent of interest only adjustable rate	64.00%
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
HELOCs [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan term	30 years
Initial draw period	10 years
Converting to amortizing loans period	20 years
Total bank loans	$ 1,602	$ 1,943
Percent of loan balance outstanding, borrowers paid only minimum due	54.00%
Home Equity Secured By Second Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total bank loans	$ 1,300