Mortgage Servicing Rights - MSRs and Related Characteristics by Portfolio (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) Multiple BasisPoint	Dec. 31, 2019 USD ($) Multiple BasisPoint	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Servicing Assets at Fair Value [Line Items]
Fair value	$ 1,978	$ 2,546	$ 1,840	$ 2,296	$ 2,458	$ 2,791
Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	211,798	223,518
Fair value	$ 1,978	$ 2,546
Value (bps) | BasisPoint	93	114
Weighted-average servicing fees (bps) | BasisPoint	32	31
Multiple (value/servicing fees) | Multiple	2.89	3.67
Weighted-average note rate	4.03%	4.17%
Weighted-average age (in years)	4 years 6 months	4 years 7 months 6 days
Weighted-average expected prepayment (constant prepayment rate)	18.70%	12.40%
Weighted-average expected life (in years)	4 years 4 months 24 days	6 years
Weighted-average option adjusted spread	6.70%	7.30%
HFA [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 41,233	$ 44,906
Fair value	$ 378	$ 486
Value (bps) | BasisPoint	92	108
Weighted-average servicing fees (bps) | BasisPoint	35	34
Multiple (value/servicing fees) | Multiple	2.64	3.15
Weighted-average note rate	4.52%	4.65%
Weighted-average age (in years)	3 years 10 months 24 days	3 years 8 months 12 days
Weighted-average expected prepayment (constant prepayment rate)	15.40%	12.20%
Weighted-average expected life (in years)	5 years 4 months 24 days	6 years 6 months
Weighted-average option adjusted spread	7.70%	8.40%
Government Insured [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 28,532	$ 35,302
Fair value	$ 312	$ 451
Value (bps) | BasisPoint	109	128
Weighted-average servicing fees (bps) | BasisPoint	40	39
Multiple (value/servicing fees) | Multiple	2.74	3.29
Weighted-average note rate	3.93%	3.99%
Weighted-average age (in years)	5 years 4 months 24 days	4 years 10 months 24 days
Weighted-average expected prepayment (constant prepayment rate)	17.40%	13.70%
Weighted-average expected life (in years)	4 years 7 months 6 days	5 years 8 months 12 days
Weighted-average option adjusted spread	7.30%	7.90%
Conventional [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 142,033	$ 143,310
Fair value	$ 1,288	$ 1,609
Value (bps) | BasisPoint	91	112
Weighted-average servicing fees (bps) | BasisPoint	30	28
Multiple (value/servicing fees) | Multiple	3.01	4.00
Weighted-average note rate	3.91%	4.07%
Weighted-average age (in years)	4 years 6 months	4 years 9 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	19.90%	12.20%
Weighted-average expected life (in years)	4 years 1 month 6 days	5 years 10 months 24 days
Weighted-average option adjusted spread	6.20%	6.90%