Mortgage Servicing Rights - MSRs and Related Characteristics by Portfolio (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) BasisPoint Multiple	Dec. 31, 2020 USD ($) BasisPoint Multiple	Jun. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Servicing Assets at Fair Value [Line Items]
Fair value	$ 2,910	$ 2,210	$ 2,713	$ 1,978	$ 1,840	$ 2,546
Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	215,973	208,955
Fair value	$ 2,910	$ 2,210
Value (bps) | BasisPoint	135	106
Weighted-average servicing fees (bps) | BasisPoint	32	32
Multiple (value/servicing fees) | Multiple	4.16	3.26
Weighted-average note rate	3.63%	3.92%
Weighted-average age (in years)	3 years 8 months 12 days	4 years 3 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	10.20%	14.40%
Weighted-average expected life (in years)	6 years 9 months 18 days	5 years 4 months 24 days
Weighted-average option adjusted spread	6.80%	6.60%
HFA [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 40,038	$ 40,396
Fair value	$ 508	$ 406
Value (bps) | BasisPoint	127	101
Weighted-average servicing fees (bps) | BasisPoint	36	35
Multiple (value/servicing fees) | Multiple	3.56	2.87
Weighted-average note rate	4.14%	4.43%
Weighted-average age (in years)	3 years 9 months 18 days	3 years 9 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	11.30%	14.10%
Weighted-average expected life (in years)	6 years 7 months 6 days	5 years 7 months 6 days
Weighted-average option adjusted spread	7.70%	7.70%
Government Insured [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 22,392	$ 25,474
Fair value	$ 304	$ 261
Value (bps) | BasisPoint	136	102
Weighted-average servicing fees (bps) | BasisPoint	41	40
Multiple (value/servicing fees) | Multiple	3.34	2.56
Weighted-average note rate	3.75%	3.91%
Weighted-average age (in years)	5 years 9 months 18 days	5 years 7 months 6 days
Weighted-average expected prepayment (constant prepayment rate)	13.40%	18.00%
Weighted-average expected life (in years)	5 years 6 months	4 years 3 months 18 days
Weighted-average option adjusted spread	7.30%	7.30%
Conventional [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 153,543	$ 143,085
Fair value	$ 2,098	$ 1,543
Value (bps) | BasisPoint	137	108
Weighted-average servicing fees (bps) | BasisPoint	30	30
Multiple (value/servicing fees) | Multiple	4.51	3.55
Weighted-average note rate	3.48%	3.78%
Weighted-average age (in years)	3 years 4 months 24 days	4 years 2 months 12 days
Weighted-average expected prepayment (constant prepayment rate)	9.40%	13.80%
Weighted-average expected life (in years)	7 years	5 years 6 months
Weighted-average option adjusted spread	6.50%	6.20%