Mortgage Servicing Rights - MSRs and Related Characteristics by Portfolio (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) BasisPoint Multiple	Dec. 31, 2017 USD ($) BasisPoint Multiple	Jun. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Servicing Assets at Fair Value [Line Items]
Fair value	$ 2,917	$ 2,645	$ 2,844	$ 2,598	$ 2,582	$ 2,591
Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	230,543	232,846
Fair value	$ 2,917	$ 2,645
Value (bps) | BasisPoint	127	114
Weighted-average servicing fees (bps) | BasisPoint	29	29
Multiple (value/servicing fees) | Multiple	4.29	3.86
Weighted-average note rate	4.12%	4.08%
Weighted-average age (in years)	4 years 2 months 12 days	4 years
Weighted-average expected prepayment (constant prepayment rate)	8.60%	10.00%
Weighted-average expected life (in years)	7 years 6 months	7 years
Weighted-average option adjusted spread	7.70%	8.00%
HFA [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 43,303	$ 40,737
Fair value	$ 527	$ 450
Value (bps) | BasisPoint	122	110
Weighted-average servicing fees (bps) | BasisPoint	34	35
Multiple (value/servicing fees) | Multiple	3.55	3.17
Weighted-average note rate	4.54%	4.43%
Weighted-average age (in years)	3 years 3 months 18 days	3 years
Weighted-average expected prepayment (constant prepayment rate)	9.00%	9.80%
Weighted-average expected life (in years)	8 years 1 month 6 days	7 years 8 months 12 days
Weighted-average option adjusted spread	8.70%	9.90%
Government Insured [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 36,258	$ 36,756
Fair value	$ 484	$ 428
Value (bps) | BasisPoint	133	116
Weighted-average servicing fees (bps) | BasisPoint	35	34
Multiple (value/servicing fees) | Multiple	3.78	3.38
Weighted-average note rate	3.96%	3.92%
Weighted-average age (in years)	4 years 6 months	4 years 3 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	10.00%	11.60%
Weighted-average expected life (in years)	7 years 1 month 6 days	6 years 6 months
Weighted-average option adjusted spread	8.30%	9.20%
Conventional [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 150,982	$ 155,353
Fair value	$ 1,906	$ 1,767
Value (bps) | BasisPoint	126	114
Weighted-average servicing fees (bps) | BasisPoint	27	27
Multiple (value/servicing fees) | Multiple	4.73	4.24
Weighted-average note rate	4.04%	4.02%
Weighted-average age (in years)	4 years 4 months 24 days	4 years 2 months 12 days
Weighted-average expected prepayment (constant prepayment rate)	8.10%	9.70%
Weighted-average expected life (in years)	7 years 6 months	6 years 10 months 24 days
Weighted-average option adjusted spread	7.20%	7.20%