Fair Values of Assets and Liabilities - Differences Between Aggregate Fair Value Carrying Amount of MLHFS for which Fair Value Option has been Elected and Aggregate Unpaid Principal Amount Contractually Obligated to Receive at Maturity (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair value carrying amount, total loans	$ 3,656	$ 4,822
Fair value carrying amount, nonaccrual loans	2	2
Fair value carrying amount, loans 90 days or more past due		1
Aggregate unpaid principal, total loans	3,550	4,763
Aggregate unpaid principal, nonaccrual loans	3	3
Aggregate unpaid principal, loans 90 days or more past due		1
Carrying amount over (under) unpaid principal, total loans	106	59
Carrying amount over (under) unpaid principal, nonaccrual loans	(1)	(1)
Carrying amount over (under) unpaid principal, loans 90 days or more past due	$ 0	$ 0