Mortgage Servicing Rights - MSRs and Related Characteristics by Portfolio (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2023	Jun. 30, 2024	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Servicing Assets at Fair Value [Line Items]
Fair value	$ 3,187	$ 3,377	$ 3,326	$ 3,582	$ 3,633	$ 3,755
MSR portfolio
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	214,993	225,338
Fair value	$ 3,187	$ 3,377
Value (bps)	1.48%	1.50%
Weighted-average servicing fees (bps)	0.30%	0.30%
Multiple (value/servicing fees)	4.92	5.00
Weighted-average note rate	4.12%	4.02%
Weighted-average age (in years)	4 years 10 months 24 days	4 years 4 months 24 days
Weighted-average expected prepayment (constant prepayment rate)	9.70%	9.60%
Weighted-average expected life (in years)	6 years 10 months 24 days	7 years
Weighted-average option adjusted spread	5.30%	4.90%
MSR portfolio | HFA
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 51,526	$ 48,286
Fair value	$ 791	$ 769
Value (bps)	1.54%	1.59%
Weighted-average servicing fees (bps)	0.36%	0.36%
Multiple (value/servicing fees)	4.32	4.45
Weighted-average note rate	4.84%	4.56%
Weighted-average age (in years)	4 years 6 months	4 years 3 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	11.00%	10.50%
Weighted-average expected life (in years)	7 years	7 years 2 months 12 days
Weighted-average option adjusted spread	5.90%	5.40%
MSR portfolio | Government
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 25,506	$ 25,996
Fair value	$ 490	$ 507
Value (bps)	1.92%	1.95%
Weighted-average servicing fees (bps)	0.45%	0.44%
Multiple (value/servicing fees)	4.30	4.41
Weighted-average note rate	4.34%	4.23%
Weighted-average age (in years)	5 years 10 months 24 days	5 years 6 months
Weighted-average expected prepayment (constant prepayment rate)	11.30%	11.10%
Weighted-average expected life (in years)	6 years 4 months 24 days	6 years 6 months
Weighted-average option adjusted spread	6.20%	5.90%
MSR portfolio | Conventional
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 137,961	$ 151,056
Fair value	$ 1,906	$ 2,101
Value (bps)	1.38%	1.39%
Weighted-average servicing fees (bps)	0.25%	0.26%
Multiple (value/servicing fees)	5.44	5.41
Weighted-average note rate	3.81%	3.81%
Weighted-average age (in years)	4 years 10 months 24 days	4 years 3 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	8.90%	9.10%
Weighted-average expected life (in years)	7 years	7 years
Weighted-average option adjusted spread	4.80%	4.60%