Fair Value Measurements - Fair Value Disclosures of Financial Instruments (Detail) - USD ($)	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014
ASSETS
Trading account assets	$ 811,000,000		$ 750,000,000	$ 965,000,000
Securities available for sale	14,376,000,000		13,360,000,000	12,245,000,000
Held-to-maturity securities	4,940,000,000		4,974,000,000	4,911,000,000
Held-to-maturity securities	4,936,000,000		5,015,000,000	4,997,000,000
Other investments	691,000,000		760,000,000	822,000,000
Loans, net of allowance			191,000,000	201,000,000
Loans, net of allowance	59,295,000,000		56,587,000,000	55,351,000,000
Loans held for sale	0		0	0
Loans held for sale	916,000,000	$ 835,000,000	734,000,000	784,000,000	$ 435,000,000
Derivative assets	793,000,000		609,000,000	413,000,000
LIABILITIES
Long-term debt	10,310,000,000		7,875,000,000	7,172,000,000
Derivative liabilities	676,000,000		784,000,000	384,000,000
Derivative liabilities, netting adjustment	0		0	0
Carrying Amount [Member]
ASSETS
Cash and short-term investments	2,434,000,000		4,922,000,000	2,993,000,000
Trading account assets	811,000,000		750,000,000	965,000,000
Securities available for sale	14,376,000,000		13,360,000,000	12,245,000,000
Held-to-maturity securities	4,936,000,000		5,015,000,000	4,997,000,000
Other investments	691,000,000		760,000,000	822,000,000
Loans, net of allowance	59,295,000,000		56,587,000,000	55,351,000,000
Loans held for sale	916,000,000		734,000,000	784,000,000
Mortgage servicing assets				308,000,000
Derivative assets	793,000,000		609,000,000	413,000,000
LIABILITIES
Deposits with no stated maturity	65,624,000,000		66,135,000,000	62,028,000,000
Time deposits	5,449,000,000		5,863,000,000	6,428,000,000
Short-term borrowings	1,084,000,000		998,000,000	1,653,000,000
Long-term debt	10,310,000,000		7,875,000,000	7,172,000,000
Derivative liabilities	676,000,000		784,000,000	384,000,000
Fair Value [Member]
ASSETS
Cash and short-term investments	2,434,000,000		4,922,000,000	2,993,000,000
Trading account assets	811,000,000		750,000,000	965,000,000
Securities available for sale	14,376,000,000		13,360,000,000	12,245,000,000
Held-to-maturity securities	4,940,000,000		4,974,000,000	4,911,000,000
Other investments	691,000,000		760,000,000	822,000,000
Loans, net of allowance	57,497,000,000		54,993,000,000	53,996,000,000
Loans held for sale	916,000,000		734,000,000	784,000,000
Mortgage servicing assets				370,000,000
Derivative assets	793,000,000		609,000,000	413,000,000
LIABILITIES
Deposits with no stated maturity	65,624,000,000		66,135,000,000	62,028,000,000
Time deposits	5,502,000,000		5,925,000,000	6,493,000,000
Short-term borrowings	677,000,000		998,000,000	1,653,000,000
Long-term debt	10,609,000,000		8,251,000,000	8,056,000,000
Derivative liabilities	676,000,000		784,000,000	384,000,000
Fair Value [Member] | Level 1 [Member]
ASSETS
Cash and short-term investments	2,434,000,000		4,922,000,000	2,993,000,000
Trading account assets	4,000,000		2,000,000	13,000,000
Securities available for sale	11,000,000		22,000,000	22,000,000
Other investments			2,000,000
Derivative assets	120,000,000		91,000,000	73,000,000
LIABILITIES
Time deposits	427,000,000		564,000,000	556,000,000
Short-term borrowings				6,000,000
Long-term debt	10,146,000,000		7,625,000,000	6,854,000,000
Derivative liabilities	102,000,000		77,000,000	55,000,000
Fair Value [Member] | Level 2 [Member]
ASSETS
Trading account assets	807,000,000		748,000,000	952,000,000
Securities available for sale	14,348,000,000		13,328,000,000	12,213,000,000
Held-to-maturity securities	4,940,000,000		4,974,000,000	4,911,000,000
Derivative assets	1,593,000,000		1,536,000,000	931,000,000
LIABILITIES
Deposits with no stated maturity	65,624,000,000		66,135,000,000	62,028,000,000
Time deposits	5,075,000,000		5,361,000,000	5,937,000,000
Short-term borrowings	677,000,000		998,000,000	1,647,000,000
Long-term debt	463,000,000		626,000,000	1,202,000,000
Derivative liabilities	1,140,000,000		1,248,000,000	721,000,000
Fair Value [Member] | Level 3 [Member]
ASSETS
Securities available for sale	17,000,000		10,000,000	10,000,000
Other investments	691,000,000		758,000,000	822,000,000
Loans, net of allowance	57,497,000,000		54,993,000,000	53,996,000,000
Loans held for sale	916,000,000		734,000,000	784,000,000
Mortgage servicing assets				370,000,000
Derivative assets	25,000,000		16,000,000	22,000,000
LIABILITIES
Derivative liabilities			1,000,000	1,000,000
Fair Value [Member] | Netting Adjustment [Member]
ASSETS
Derivative assets, netting adjustment	(945,000,000)		(1,034,000,000)	(613,000,000)
LIABILITIES
Derivative liabilities, netting adjustment	$ (566,000,000)		$ (542,000,000)	$ (393,000,000)