Mortgage Servicing Rights - MSRs and Related Characteristics by Portfolio (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) BasisPoint Multiple	Dec. 31, 2018 USD ($) BasisPoint Multiple	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Servicing Assets at Fair Value [Line Items]
Fair value	$ 2,656	$ 2,791	$ 2,780	$ 2,645
Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	228,279	229,284
Fair value	$ 2,656	$ 2,791
Value (bps) | BasisPoint	116	122
Weighted-average servicing fees (bps) | BasisPoint	30	30
Multiple (value/servicing fees) | Multiple	3.91	4.11
Weighted-average note rate	4.17%	4.15%
Weighted-average age (in years)	4 years 6 months	4 years 3 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	10.70%	9.50%
Weighted-average expected life (in years)	6 years 8 months 12 days	7 years 2 months 12 days
Weighted-average option adjusted spread	7.60%	7.60%
HFA [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 45,133	$ 44,384
Fair value	$ 510	$ 526
Value (bps) | BasisPoint	113	119
Weighted-average servicing fees (bps) | BasisPoint	34	34
Multiple (value/servicing fees) | Multiple	3.30	3.45
Weighted-average note rate	4.63%	4.59%
Weighted-average age (in years)	3 years 4 months 24 days	3 years 3 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	10.80%	9.80%
Weighted-average expected life (in years)	7 years 2 months 12 days	7 years 8 months 12 days
Weighted-average option adjusted spread	8.60%	8.60%
Government Insured [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 35,785	$ 35,990
Fair value	$ 451	$ 465
Value (bps) | BasisPoint	126	129
Weighted-average servicing fees (bps) | BasisPoint	36	36
Multiple (value/servicing fees) | Multiple	3.49	3.63
Weighted-average note rate	3.99%	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)	12.00%	11.00%
Weighted-average expected life (in years)	6 years 3 months 18 days	6 years 8 months 12 days
Weighted-average option adjusted spread	8.20%	8.30%
Conventional [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 147,361	$ 148,910
Fair value	$ 1,695	$ 1,800
Value (bps) | BasisPoint	115	121
Weighted-average servicing fees (bps) | BasisPoint	27	27
Multiple (value/servicing fees) | Multiple	4.28	4.52
Weighted-average note rate	4.07%	4.06%
Weighted-average age (in years)	4 years 8 months 12 days	4 years 6 months
Weighted-average expected prepayment (constant prepayment rate)	10.30%	9.10%
Weighted-average expected life (in years)	6 years 7 months 6 days	7 years 1 month 6 days
Weighted-average option adjusted spread	7.10%	7.20%