Mortgage Servicing Rights - MSRs and Related Characteristics by Portfolio (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($) BasisPoint Multiple	Dec. 31, 2020 USD ($) Multiple BasisPoint	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Servicing Assets at Fair Value [Line Items]
Fair value	$ 2,787	$ 2,210	$ 1,887	$ 2,546
Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	208,662	208,955
Fair value	$ 2,787	$ 2,210
Value (bps) | BasisPoint	134	106
Weighted-average servicing fees (bps) | BasisPoint	32	32
Multiple (value/servicing fees) | Multiple	4.12	3.26
Weighted-average note rate	3.80%	3.92%
Weighted-average age (in years)	4 years	4 years 3 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	10.30%	14.40%
Weighted-average expected life (in years)	6 years 8 months 12 days	5 years 4 months 24 days
Weighted-average option adjusted spread	6.70%	6.60%
HFA [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 39,757	$ 40,396
Fair value	$ 499	$ 406
Value (bps) | BasisPoint	126	101
Weighted-average servicing fees (bps) | BasisPoint	35	35
Multiple (value/servicing fees) | Multiple	3.56	2.87
Weighted-average note rate	4.33%	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)	11.20%	14.10%
Weighted-average expected life (in years)	6 years 9 months 18 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	$ 23,734	$ 25,474
Fair value	$ 312	$ 261
Value (bps) | BasisPoint	131	102
Weighted-average servicing fees (bps) | BasisPoint	40	40
Multiple (value/servicing fees) | Multiple	3.31	2.56
Weighted-average note rate	3.88%	3.91%
Weighted-average age (in years)	5 years 10 months 24 days	5 years 7 months 6 days
Weighted-average expected prepayment (constant prepayment rate)	13.30%	18.00%
Weighted-average expected life (in years)	5 years 7 months 6 days	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	$ 145,171	$ 143,085
Fair value	$ 1,976	$ 1,543
Value (bps) | BasisPoint	136	108
Weighted-average servicing fees (bps) | BasisPoint	30	30
Multiple (value/servicing fees) | Multiple	4.47	3.55
Weighted-average note rate	3.64%	3.78%
Weighted-average age (in years)	3 years 9 months 18 days	4 years 2 months 12 days
Weighted-average expected prepayment (constant prepayment rate)	9.50%	13.80%
Weighted-average expected life (in years)	6 years 10 months 24 days	5 years 6 months
Weighted-average option adjusted spread	6.30%	6.20%