Fair Value Of Financial Instruments (Aggregate Fair Value And Remaining Contractual Principal Balance Outstanding On Fair Value Option) (Detail) - USD ($) $ in Millions	Jun. 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	$ 819	$ 2,296
Loans receivable, fair value	683	1,679
Loans receivable, difference	135	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	649	732
Loans receivable, 90 days or more past due, contractual outstanding principal	170	170
Loans receivable, fair value	645	727
Loans receivable, 90 days or more past due, fair value	38	32
Loans receivable, difference	3	5
Loans receivable, 90 days or more past due, difference	132	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	175	180
Long-term debt instruments, fair value	149	115
Long-term debt instruments, difference	27	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	1,791	1,882
Long-term debt instruments, fair value	981	1,069
Long-term debt instruments, difference	$ 810	$ 813