Fair Value Measurements (Fair Value Measurements - Additional Information) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Net fair value of the interest rate lock commitments		$ 8
Change in the fair value of the interest rate lock commitments, due to decrease in current interest rates of 25 bp		3
Change in the fair value of the interest rate lock commitments, due to decrease in current interest rates of 50 bp		7
Change in the fair value of the interest rate lock commitments, due to increase in current interest rates of 25 bp		4
Change in the fair value of the interest rate lock commitments, due to increase in current interest rates of 50 bp		8
Change in fair value of interest rate lock commitments, due to 10% adverse changes in the assumed loan closing rates		1
Change in fair value of interest rate lock commitments, due to 20% adverse changes in the assumed loan closing rates		2
Change in fair value of interest rate lock commitments, due to 10% favorable changes in the assumed loan closing rates		1
Change in fair value of interest rate lock commitments, due to 20% favorable changes in the assumed loan closing rates		2
Existing loans held for sale, further adjusted		399	$ 686
Net Losses On Operating Lease Equipment		42	9
OTTI	[1]	54	16	$ 5
Larger commercial loans, subject to impairment review		1
Asset Impairment Charges		277	262
Commercial Loans Held For Sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Existing loans held for sale, further adjusted			2
Net impact related to fair value adjustments		31	30
Gain Loss On Sales Of Loans Net		2
Other Real Estate Owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Net impact related to fair value adjustments		6	9
Nonrecurring Losses Included As Charge-Offs		4	8
Private equity investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
OTTI		$ 1	$ 9

[1] (a) Included in securities gains, net, in the Consolidated Statements of Income.