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, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair value carrying amount, total loans	$ 3,852	$ 2,035
Fair value carrying amount, nonaccrual loans	1	2
Aggregate unpaid principal, total loans	3,729	1,972
Aggregate unpaid principal, nonaccrual loans	1	2
Carrying amount over (under) unpaid principal, total loans	$ 123	$ 63