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	Mar. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair value carrying amount, total loans	$ 2,690	$ 2,035
Fair value carrying amount, nonaccrual loans	2	2
Fair value carrying amount, loans 90 days or more past due	0	0
Aggregate unpaid principal, total loans	2,596	1,972
Aggregate unpaid principal, nonaccrual loans	2	2
Aggregate unpaid principal, loans 90 days or more past due	0	0
Carrying amount over (under) unpaid principal, total loans	94	63
Carrying amount over (under) unpaid principal, nonaccrual loans	0	0
Carrying amount over (under) unpaid principal, loans 90 days or more past due	$ 0	$ 0