Mortgage Servicing Rights - Sensitivity to Changes in Interest Rates of the Fair Value of MSRs Portfolio and Related Derivative Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 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	$ (433)	$ (520)
Net fair value 50 basis points	(192)	(231)
Net fair value 25 basis points	(90)	(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	399	453
Net fair value 50 basis points	187	216
Net fair value 25 basis points	90	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	(34)	(67)
Net fair value 50 basis points	(5)	(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	240	302
Net fair value 50 basis points	142	177
Net fair value 25 basis points	77	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	(277)	(336)
Net fair value 50 basis points	(154)	(184)
Net fair value 25 basis points	(82)	(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	(37)	(34)
Net fair value 50 basis points	(12)	(7)
Net fair value 25 basis points	$ (5)	$ (1)