Mortgage Servicing Rights - MSRs and Related Characteristics by Portfolio (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) BasisPoint Multiple	Dec. 31, 2017 USD ($) BasisPoint Multiple	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Servicing Assets at Fair Value [Line Items]
Fair value	$ 2,780	$ 2,645	$ 2,642	$ 2,591
Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	233,052	232,846
Fair value	$ 2,780	$ 2,645
Value (bps) | BasisPoint	119	114
Weighted-average servicing fees (bps) | BasisPoint	29	29
Multiple (value/servicing fees) | Multiple	4.05	3.86
Weighted-average note rate	4.08%	4.08%
Weighted-average age (in years)	4 years 1 month 6 days	4 years
Weighted-average expected prepayment (constant prepayment rate)	9.20%	10.00%
Weighted-average expected life (in years)	7 years 3 months 18 days	7 years
Weighted-average option adjusted spread	7.80%	8.00%
HFA [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 41,815	$ 40,737
Fair value	$ 486	$ 450
Value (bps) | BasisPoint	116	110
Weighted-average servicing fees (bps) | BasisPoint	35	35
Multiple (value/servicing fees) | Multiple	3.36	3.17
Weighted-average note rate	4.46%	4.43%
Weighted-average age (in years)	3 years 1 month 6 days	3 years
Weighted-average expected prepayment (constant prepayment rate)	9.30%	9.80%
Weighted-average expected life (in years)	7 years 10 months 24 days	7 years 8 months 12 days
Weighted-average option adjusted spread	8.80%	9.90%
Government Insured [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 37,055	$ 36,756
Fair value	$ 461	$ 428
Value (bps) | BasisPoint	124	116
Weighted-average servicing fees (bps) | BasisPoint	35	34
Multiple (value/servicing fees) | Multiple	3.59	3.38
Weighted-average note rate	3.93%	3.92%
Weighted-average age (in years)	4 years 3 months 18 days	4 years 3 months 18 days
Weighted-average expected prepayment (constant prepayment rate)	10.80%	11.60%
Weighted-average expected life (in years)	6 years 9 months 18 days	6 years 6 months
Weighted-average option adjusted spread	8.40%	9.20%
Conventional [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 154,182	$ 155,353
Fair value	$ 1,833	$ 1,767
Value (bps) | BasisPoint	119	114
Weighted-average servicing fees (bps) | BasisPoint	27	27
Multiple (value/servicing fees) | Multiple	4.44	4.24
Weighted-average note rate	4.02%	4.02%
Weighted-average age (in years)	4 years 3 months 18 days	4 years 2 months 12 days
Weighted-average expected prepayment (constant prepayment rate)	8.80%	9.70%
Weighted-average expected life (in years)	7 years 3 months 18 days	6 years 10 months 24 days
Weighted-average option adjusted spread	7.30%	7.20%