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	Sep. 30, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair value carrying amount, total loans	$ 7,314	$ 5,533
Fair value carrying amount, nonaccrual loans	1	1
Fair value carrying amount, loans 90 days or more past due		1
Aggregate unpaid principal, total loans	6,983	5,366
Aggregate unpaid principal, nonaccrual loans	1	1
Aggregate unpaid principal, loans 90 days or more past due		1
Carrying amount over (under) unpaid principal, total loans	$ 331	$ 167