Mortgage Servicing Rights - Sensitivity to Changes in Interest Rates to Fair Value of MSRs Portfolio and Related Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Down Scenario [Member] | MSR Portfolio [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	$ (494)	$ (370)
Net fair value 50 basis points	(223)	(217)
Net fair value 25 basis points	(107)	(118)
Down Scenario [Member] | Derivative Instrument Hedges [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	427	473
Net fair value 50 basis points	211	249
Net fair value 25 basis points	102	124
Down Scenario [Member] | Net Sensitivity [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	(67)	103
Net fair value 50 basis points	(12)	32
Net fair value 25 basis points	(5)	6
Up Scenario [Member] | MSR Portfolio [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	324	480
Net fair value 50 basis points	173	249
Net fair value 25 basis points	92	126
Up Scenario [Member] | Derivative Instrument Hedges [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	(317)	(486)
Net fair value 50 basis points	(174)	(243)
Net fair value 25 basis points	(93)	(121)
Up Scenario [Member] | Net Sensitivity [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	7	(6)
Net fair value 50 basis points	(1)	6
Net fair value 25 basis points	$ (1)	$ 5