Fair Value Measurements (Fair Values of Assets and Liabilities (Significant Unobservable Level 3 Inputs Nonrecurring Basis)) (Detail) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Commercial Loans Held For Sale | Commercial
Quantitative Information About Level 3 Fair Value Measurements
Fair value measurements nonrecurring assets	$ 4	$ 45
Commercial and Industrial Loans | Commercial
Quantitative Information About Level 3 Fair Value Measurements
Fair value measurements nonrecurring assets	161	405
Commercial Mortgage Loans | Commercial
Quantitative Information About Level 3 Fair Value Measurements
Fair value measurements nonrecurring assets	3	26
Commercial Leases | Commercial
Quantitative Information About Level 3 Fair Value Measurements
Fair value measurements nonrecurring assets	14	3
OREO Property
Quantitative Information About Level 3 Fair Value Measurements
Fair value measurements nonrecurring assets	17	11
Bank premises and equipment
Quantitative Information About Level 3 Fair Value Measurements
Fair value measurements nonrecurring assets	37	16
Operating lease equipment
Quantitative Information About Level 3 Fair Value Measurements
Fair value measurements nonrecurring assets	10	$ 56
Private Equity Investments
Quantitative Information About Level 3 Fair Value Measurements
Comparable company analysis, fair value	3
Liquidity discount, fair value	$ 28
Minimum | Private Equity Investments
Quantitative Information About Level 3 Fair Value Measurements
Liquidity discount	0.00%
Maximum | Private Equity Investments
Quantitative Information About Level 3 Fair Value Measurements
Liquidity discount	43.00%
Weighted average | Commercial Loans Held For Sale
Quantitative Information About Level 3 Fair Value Measurements
Cost to sell	10.00%	10.00%
Weighted average | Private Equity Investments
Quantitative Information About Level 3 Fair Value Measurements
Liquidity discount	13.70%