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, 2012	Dec. 31, 2011
Down Scenario | MSR portfolio
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	$ (262)	$ (305)
Net fair value 50 basis points	(162)	(183)
Net fair value 25 basis points	(94)	(98)
Down Scenario | Derivative instrument hedges
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	405	378
Net fair value 50 basis points	208	204
Net fair value 25 basis points	105	104
Down Scenario | Net sensitivity
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	143	73
Net fair value 50 basis points	46	21
Net fair value 25 basis points	11	6
Up Scenario | MSR portfolio
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	483	460
Net fair value 50 basis points	231	223
Net fair value 25 basis points	109	107
Up Scenario | Derivative instrument hedges
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	(422)	(445)
Net fair value 50 basis points	(210)	(217)
Net fair value 25 basis points	(105)	(107)
Up Scenario | Net sensitivity
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	61	15
Net fair value 50 basis points	21	6
Net fair value 25 basis points	$ 4