Mortgage Servicing Rights - MSRs and Related Characteristics by Portfolio (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($) BasisPoint Multiple	Dec. 31, 2019 USD ($) Multiple BasisPoint	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Servicing Assets at Fair Value [Line Items]
Fair value	$ 1,887	$ 2,546	$ 2,656	$ 2,791
Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	224,120	223,518
Fair value	$ 1,887	$ 2,546
Value (bps) | BasisPoint	84	114
Weighted-average servicing fees (bps) | BasisPoint	31	31
Multiple (value/servicing fees) | Multiple	2.68	3.67
Weighted-average note rate	4.15%	4.17%
Weighted-average age (in years)	4 years 7 months 6 days	4 years 7 months 6 days
Weighted-average expected prepayment (constant prepayment rate)	19.60%	12.40%
Weighted-average expected life (in years)	4 years 1 month 6 days	6 years
Weighted-average option adjusted spread	7.30%	7.30%
HFA [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 45,073	$ 44,906
Fair value	$ 396	$ 486
Value (bps) | BasisPoint	88	108
Weighted-average servicing fees (bps) | BasisPoint	34	34
Multiple (value/servicing fees) | Multiple	2.55	3.15
Weighted-average note rate	4.63%	4.65%
Weighted-average age (in years)	3 years 9 months 18 days	3 years 8 months 12 days
Weighted-average expected prepayment (constant prepayment rate)	16.20%	12.20%
Weighted-average expected life (in years)	5 years 1 month 6 days	6 years 6 months
Weighted-average option adjusted spread	8.30%	8.40%
Government Insured [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 34,836	$ 35,302
Fair value	$ 349	$ 451
Value (bps) | BasisPoint	100	128
Weighted-average servicing fees (bps) | BasisPoint	39	39
Multiple (value/servicing fees) | Multiple	2.54	3.29
Weighted-average note rate	3.97%	3.99%
Weighted-average age (in years)	5 years	4 years 10 months 24 days
Weighted-average expected prepayment (constant prepayment rate)	19.50%	13.70%
Weighted-average expected life (in years)	4 years 2 months 12 days	5 years 8 months 12 days
Weighted-average option adjusted spread	7.80%	7.90%
Conventional [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 144,211	$ 143,310
Fair value	$ 1,142	$ 1,609
Value (bps) | BasisPoint	79	112
Weighted-average servicing fees (bps) | BasisPoint	29	28
Multiple (value/servicing fees) | Multiple	2.77	4.00
Weighted-average note rate	4.05%	4.07%
Weighted-average age (in years)	4 years 9 months 18 days	4 years 9 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	20.70%	12.20%
Weighted-average expected life (in years)	3 years 9 months 18 days	5 years 10 months 24 days
Weighted-average option adjusted spread	6.70%	6.90%