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	Mar. 31, 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	$ (454)	$ (370)
Net fair value 50 basis points	(243)	(217)
Net fair value 25 basis points	(124)	(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	487	473
Net fair value 50 basis points	249	249
Net fair value 25 basis points	123	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	33	103
Net fair value 50 basis points	6	32
Net fair value 25 basis points	(1)	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	428	480
Net fair value 50 basis points	229	249
Net fair value 25 basis points	122	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	(397)	(486)
Net fair value 50 basis points	(217)	(243)
Net fair value 25 basis points	(115)	(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	31	(6)
Net fair value 50 basis points	12	6
Net fair value 25 basis points	$ 7	$ 5