Mortgage Servicing Rights - MSRs and Related Characteristics by Portfolio (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) BasisPoint Multiple	Dec. 31, 2018 USD ($) BasisPoint Multiple	Jun. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Servicing Assets at Fair Value [Line Items]
Fair value	$ 2,296	$ 2,791	$ 2,458	$ 2,917	$ 2,844	$ 2,645
Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	225,399	229,284
Fair value	$ 2,296	$ 2,791
Value (bps) | BasisPoint	102	122
Weighted-average servicing fees (bps) | BasisPoint	31	30
Multiple (value/servicing fees) | Multiple	3.33	4.11
Weighted-average note rate	4.18%	4.15%
Weighted-average age (in years)	4 years 7 months 6 days	4 years 3 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	14.60%	9.50%
Weighted-average expected life (in years)	5 years 4 months 24 days	7 years 2 months 12 days
Weighted-average option adjusted spread	7.40%	7.60%
HFA [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 44,968	$ 44,384
Fair value	$ 453	$ 526
Value (bps) | BasisPoint	101	119
Weighted-average servicing fees (bps) | BasisPoint	34	34
Multiple (value/servicing fees) | Multiple	2.94	3.45
Weighted-average note rate	4.66%	4.59%
Weighted-average age (in years)	3 years 7 months 6 days	3 years 3 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	13.60%	9.80%
Weighted-average expected life (in years)	6 years	7 years 8 months 12 days
Weighted-average option adjusted spread	8.40%	8.60%
Government Insured [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 35,634	$ 35,990
Fair value	$ 414	$ 465
Value (bps) | BasisPoint	116	129
Weighted-average servicing fees (bps) | BasisPoint	38	36
Multiple (value/servicing fees) | Multiple	3.05	3.63
Weighted-average note rate	4.01%	3.97%
Weighted-average age (in years)	4 years 9 months 18 days	4 years 8 months 12 days
Weighted-average expected prepayment (constant prepayment rate)	15.40%	11.00%
Weighted-average expected life (in years)	5 years 2 months 12 days	6 years 8 months 12 days
Weighted-average option adjusted spread	8.00%	8.30%
Conventional [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 144,797	$ 148,910
Fair value	$ 1,429	$ 1,800
Value (bps) | BasisPoint	99	121
Weighted-average servicing fees (bps) | BasisPoint	28	27
Multiple (value/servicing fees) | Multiple	3.57	4.52
Weighted-average note rate	4.08%	4.06%
Weighted-average age (in years)	4 years 9 months 18 days	4 years 6 months
Weighted-average expected prepayment (constant prepayment rate)	14.70%	9.10%
Weighted-average expected life (in years)	5 years 2 months 12 days	7 years 1 month 6 days
Weighted-average option adjusted spread	7.00%	7.20%