Mortgage Servicing Rights - Sensitivity to Changes in Interest Rates of the Fair Value of MSR Portfolio and Related Derivative Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
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	$ (420)	$ (520)
Net fair value 50 basis points	(184)	(231)
Net fair value 25 basis points	(85)	(109)
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	405	453
Net fair value 50 basis points	182	216
Net fair value 25 basis points	85	105
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	(15)	(67)
Net fair value 50 basis points	(2)	(15)
Net fair value 25 basis points		(4)
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	228	302
Net fair value 50 basis points	134	177
Net fair value 25 basis points	73	95
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	(247)	(336)
Net fair value 50 basis points	(138)	(184)
Net fair value 25 basis points	(74)	(96)
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	(19)	(34)
Net fair value 50 basis points	(4)	(7)
Net fair value 25 basis points	$ (1)	$ (1)