Loans Mortgages With Potential Concentration of Credit Risk (Details) $ in Millions	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current Weighted Average FICO Score on Mortgages With Potential Concentration of Credit Risk	756	751
Fair Value, Concentration of Risk, Loans Receivable	$ 10,843	$ 10,322
Residential Mortgage Interest Only Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value, Concentration of Risk, Loans Receivable	646	1,124
Residential Mortgage Interest Only Loans [Member] | Mortgages With Mortgage Insurance or With LTV Ratio Less Than or Equal to 80% [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value, Concentration of Risk, Loans Receivable	569	845
Residential Mortgage Interest Only Loans [Member] | Mortgages With No Mortgage Insurance and With LTV Ratio Greater Than 80% [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value, Concentration of Risk, Loans Receivable	77	279
Residential Mortgage Amortizing Loans [Member] | Mortgages With LTV Ratio Greater Than 80% and/or second liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair Value, Concentration of Risk, Loans Receivable	$ 10,197	$ 9,198