Fair Value Of Financial Instruments (Aggregate Fair Value And Remaining Contractual Principal Balance Outstanding On Fair Value Option) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Loans receivable, contractual outstanding principal	$ 555	$ 2,296
Loans receivable, fair value	428	1,679
Loans receivable, difference	127	617
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	390	732
Loans receivable, 90 days or more past due, contractual outstanding principal	165	170
Loans receivable, fair value	389	727
Loans receivable, 90 days or more past due, fair value	39	32
Loans receivable, difference	1	5
Loans receivable, 90 days or more past due, difference	126	138
Corporate Loans [Member] | Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Loans receivable, 90 days or more past due, contractual outstanding principal	0	1,394
Loans receivable, 90 days or more past due, fair value	0	920
Loans receivable, 90 days or more past due, difference	0	474
Medium Term Notes [Member] | Non Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Long-term debt instruments, contractual outstanding principal	139	180
Long-term debt instruments, fair value	123	115
Long-term debt instruments, difference	16	65
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Long-term debt instruments, contractual outstanding principal	1,525	1,882
Long-term debt instruments, fair value	709	1,069
Long-term debt instruments, difference	$ 816	$ 813