Mortgage Servicing Rights - Sensitivity to Changes in Market Interest Rates to Fair Value of MSRs Portfolio and Related Derivative Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Down Scenario [Member] | Mortgage Servicing Rights [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	$ (585)	$ (540)
Net fair value 50 basis points	(272)	(242)
Net fair value 25 basis points	(129)	(114)
Down Scenario [Member] | Derivative [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	496	441
Net fair value 50 basis points	247	223
Net fair value 25 basis points	121	109
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	(89)	(99)
Net fair value 50 basis points	(25)	(19)
Net fair value 25 basis points	(8)	(5)
Up Scenario [Member] | Mortgage Servicing Rights [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	375	346
Net fair value 50 basis points	200	185
Net fair value 25 basis points	108	100
Up Scenario [Member] | Derivative [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	(368)	(375)
Net fair value 50 basis points	(203)	(197)
Net fair value 25 basis points	(108)	(102)
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	(29)
Net fair value 50 basis points	$ (3)	(12)
Net fair value 25 basis points		$ (2)