Fair Value Measurements (Fair Values of Assets and Liabilities (Significant Unobservable Level 3 Inputs Nonrecurring Basis)) (Detail) - Commercial - USD ($) $ in Millions	Mar. 31, 2020	Mar. 31, 2019
Commercial loans held for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value measurements nonrecurring assets	$ 16
Commercial and Industrial Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value measurements nonrecurring assets	141	$ 109
Commercial mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value measurements nonrecurring assets	45	9
Commercial Leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value measurements nonrecurring assets	8	13
Consumer Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value measurements nonrecurring assets	124
OREO Property
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value measurements nonrecurring assets	17	12
Bank premises and equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value measurements nonrecurring assets	8	22
Operating lease equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value measurements nonrecurring assets	10
Private equity investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value measurements nonrecurring assets	$ 70	$ 7