Fair Value Of Financial Instruments (Aggregate Fair Value And Remaining Contractual Principal Balance Outstanding On Fair Value Option) (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Non Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Medium-term notes, contractual outstanding principal	$ 177	$ 158
Medium-term notes, fair value	127	101
Medium-term notes, difference	51	57
Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Loans receivable, contractual outstanding principal	1,796	1,258
Variable interest entity notes, contractual outstanding principal	1,941	2,449
Loans receivable, fair value	1,632	1,066
Variable interest entity notes, fair value	1,140	1,351
Loans receivable, difference	163	192
Variable interest entity notes, difference	801	1,098
Residential Mortgage Loans [Member] | Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Loans receivable, contractual outstanding principal	766	965
Loans receivable, 90 days or more past due, contractual outstanding principal	157	143
Loans receivable, fair value	721	894
Loans receivable, 90 days or more past due, fair value	38	22
Loans receivable, difference	44	71
Loans receivable, 90 days or more past due, difference	119	121
Corporate Loans [Member] | Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Loans receivable, contractual outstanding principal	873	150
Loans receivable, fair value	873	150
Loans receivable, difference	$ 0	$ 0