Loans - Commercial Banking: Risk Profile by Geographic Region and Internal Risk Rating (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 227,613	$ 229,851	$ 203,978
Commercial Banking
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 64,241	$ 63,266
Percentage of portfolio	100.00%	100.00%
Commercial Banking | Geographic Concentration Risk | Loans Receivable | Northeast
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 24,394	$ 24,399
Percentage of portfolio	38.00%	38.60%
Commercial Banking | Geographic Concentration Risk | Loans Receivable | Mid-Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 6,310	$ 5,832
Percentage of portfolio	9.80%	9.20%
Commercial Banking | Geographic Concentration Risk | Loans Receivable | South
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 19,394	$ 19,350
Percentage of portfolio	30.20%	30.60%
Commercial Banking | Geographic Concentration Risk | Loans Receivable | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 14,143	$ 13,685
Percentage of portfolio	22.00%	21.60%
Commercial Banking | Internal Risk Rating Concentration Risk | Loans Receivable | Noncriticized
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 59,663	$ 59,743
Percentage of portfolio	92.90%	94.40%
Commercial Banking | Internal Risk Rating Concentration Risk | Loans Receivable | Criticized | Criticized performing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 2,595	$ 2,015
Percentage of portfolio	4.00%	3.20%
Commercial Banking | Internal Risk Rating Concentration Risk | Loans Receivable | Criticized | Criticized nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 1,050	$ 550
Percentage of portfolio	1.60%	0.90%
Commercial Banking | Commercial and multifamily real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 25,559	$ 25,518
Percentage of portfolio	100.00%	100.00%
Commercial Banking | Commercial and multifamily real estate | Geographic Concentration Risk | Loans Receivable | Northeast
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 15,312	$ 15,949
Percentage of portfolio	59.80%	62.50%
Commercial Banking | Commercial and multifamily real estate | Geographic Concentration Risk | Loans Receivable | Mid-Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 3,032	$ 2,797
Percentage of portfolio	11.90%	11.00%
Commercial Banking | Commercial and multifamily real estate | Geographic Concentration Risk | Loans Receivable | South
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 4,054	$ 4,070
Percentage of portfolio	15.90%	15.90%
Commercial Banking | Commercial and multifamily real estate | Geographic Concentration Risk | Loans Receivable | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 3,161	$ 2,702
Percentage of portfolio	12.40%	10.60%
Commercial Banking | Commercial and multifamily real estate | Internal Risk Rating Concentration Risk | Loans Receivable | Noncriticized
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 25,219	$ 25,130
Percentage of portfolio	98.70%	98.50%
Commercial Banking | Commercial and multifamily real estate | Internal Risk Rating Concentration Risk | Loans Receivable | Criticized | Criticized performing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 278	$ 350
Percentage of portfolio	1.10%	1.40%
Commercial Banking | Commercial and multifamily real estate | Internal Risk Rating Concentration Risk | Loans Receivable | Criticized | Criticized nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 30	$ 7
Percentage of portfolio	0.10%	0.00%
Commercial Banking | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 38,102	$ 37,135
Percentage of portfolio	100.00%	100.00%
Commercial Banking | Commercial and industrial | Geographic Concentration Risk | Loans Receivable | Northeast
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 8,724	$ 8,074
Percentage of portfolio	22.90%	21.80%
Commercial Banking | Commercial and industrial | Geographic Concentration Risk | Loans Receivable | Mid-Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 3,255	$ 3,010
Percentage of portfolio	8.50%	8.10%
Commercial Banking | Commercial and industrial | Geographic Concentration Risk | Loans Receivable | South
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 15,303	$ 15,240
Percentage of portfolio	40.20%	41.00%
Commercial Banking | Commercial and industrial | Geographic Concentration Risk | Loans Receivable | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 10,820	$ 10,811
Percentage of portfolio	28.40%	29.10%
Commercial Banking | Commercial and industrial | Internal Risk Rating Concentration Risk | Loans Receivable | Noncriticized
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 33,872	$ 34,008
Percentage of portfolio	88.90%	91.60%
Commercial Banking | Commercial and industrial | Internal Risk Rating Concentration Risk | Loans Receivable | Criticized | Criticized performing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 2,315	$ 1,662
Percentage of portfolio	6.00%	4.50%
Commercial Banking | Commercial and industrial | Internal Risk Rating Concentration Risk | Loans Receivable | Criticized | Criticized nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 1,014	$ 538
Percentage of portfolio	2.70%	1.40%
Commercial Banking | Small-ticket commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 580	$ 613
Percentage of portfolio	100.00%	100.00%
Commercial Banking | Small-ticket commercial real estate | Geographic Concentration Risk | Loans Receivable | Northeast
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 358	$ 376
Percentage of portfolio	61.60%	61.30%
Commercial Banking | Small-ticket commercial real estate | Geographic Concentration Risk | Loans Receivable | Mid-Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 23	$ 25
Percentage of portfolio	4.00%	4.10%
Commercial Banking | Small-ticket commercial real estate | Geographic Concentration Risk | Loans Receivable | South
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 37	$ 40
Percentage of portfolio	6.40%	6.50%
Commercial Banking | Small-ticket commercial real estate | Geographic Concentration Risk | Loans Receivable | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 162	$ 172
Percentage of portfolio	28.00%	28.10%
Commercial Banking | Small-ticket commercial real estate | Internal Risk Rating Concentration Risk | Loans Receivable | Noncriticized
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 572	$ 605
Percentage of portfolio	98.50%	98.70%
Commercial Banking | Small-ticket commercial real estate | Internal Risk Rating Concentration Risk | Loans Receivable | Criticized | Criticized performing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 2	$ 3
Percentage of portfolio	0.40%	0.50%
Commercial Banking | Small-ticket commercial real estate | Internal Risk Rating Concentration Risk | Loans Receivable | Criticized | Criticized nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 6	$ 5
Percentage of portfolio	1.10%	0.80%
Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 18,568	$ 19,518
Acquired Loans | Commercial Banking
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	933	958
Acquired Loans | Commercial Banking | Internal Risk Rating Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 933	$ 958
Percentage of portfolio	1.50%	1.50%
Acquired Loans | Commercial Banking | Internal Risk Rating Concentration Risk | Loans Receivable | Noncriticized
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 204	$ 128
Acquired Loans | Commercial Banking | Internal Risk Rating Concentration Risk | Loans Receivable | Criticized | Criticized performing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	694	793
Acquired Loans | Commercial Banking | Internal Risk Rating Concentration Risk | Loans Receivable | Criticized | Criticized nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	35	37
Acquired Loans | Commercial Banking | Commercial and multifamily real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	32	31
Acquired Loans | Commercial Banking | Commercial and multifamily real estate | Internal Risk Rating Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 32	$ 31
Percentage of portfolio	0.10%	0.10%
Acquired Loans | Commercial Banking | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 901	$ 927
Acquired Loans | Commercial Banking | Commercial and industrial | Internal Risk Rating Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 901	$ 927
Percentage of portfolio	2.40%	2.50%
Acquired Loans | Commercial Banking | Small-ticket commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 0	$ 0
Acquired Loans | Commercial Banking | Small-ticket commercial real estate | Internal Risk Rating Concentration Risk | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 0	$ 0
Percentage of portfolio	0.00%	0.00%