Mortgage Servicing Rights - Sensitivity to Changes in Interest Rates of the Fair Value of MSR Portfolio and Related Derivative Instruments (Detail) - USD ($) $ in Millions	Jun. 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	$ (469)	$ (520)
Net fair value 50 basis points	(207)	(231)
Net fair value 25 basis points	(97)	(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	422	453
Net fair value 50 basis points	197	216
Net fair value 25 basis points	94	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	(47)	(67)
Net fair value 50 basis points	(10)	(15)
Net fair value 25 basis points	(3)	(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	263	302
Net fair value 50 basis points	154	177
Net fair value 25 basis points	84	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	(290)	(336)
Net fair value 50 basis points	(160)	(184)
Net fair value 25 basis points	(85)	(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	(27)	(34)
Net fair value 50 basis points	(6)	(7)
Net fair value 25 basis points	$ (1)	$ (1)