Mortgage Servicing Rights - MSRs and Related Characteristics by Portfolio (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) BasisPoint Multiple	Dec. 31, 2014 USD ($) BasisPoint Multiple	Jun. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)
Servicing Assets at Fair Value [Line Items]
Fair value	$ 2,397	$ 2,338	$ 2,481	$ 2,461	$ 2,412	$ 2,680
Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	226,935	222,797
Fair value	$ 2,397	$ 2,338
Value (bps) | BasisPoint	106	105
Weighted-average servicing fees (bps) | BasisPoint	29	29
Multiple (value/servicing fees) | Multiple	3.66	3.62
Weighted-average note rate	4.13%	4.19%
Weighted-average age (in years)	3 years 4 months 24 days	3 years 2 months 12 days
Weighted-average expected prepayment (constant prepayment rate)	12.20%	12.10%
Weighted-average expected life (in years)	6 years 1 month 6 days	6 years 3 months 18 days
Weighted-average discount rate	10.00%	10.10%
HFA [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 23,843	$ 19,706
Fair value	$ 265	$ 213
Value (bps) | BasisPoint	111	108
Weighted-average servicing fees (bps) | BasisPoint	36	37
Multiple (value/servicing fees) | Multiple	3.08	2.92
Weighted-average note rate	4.47%	4.58%
Weighted-average age (in years)	3 years 3 months 18 days	3 years 7 months 6 days
Weighted-average expected prepayment (constant prepayment rate)	12.70%	12.80%
Weighted-average expected life (in years)	6 years 1 month 6 days	6 years 2 months 12 days
Weighted-average discount rate	11.80%	11.90%
Government [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 40,362	$ 40,471
Fair value	$ 433	$ 426
Value (bps) | BasisPoint	107	105
Weighted-average servicing fees (bps) | BasisPoint	33	33
Multiple (value/servicing fees) | Multiple	3.24	3.18
Weighted-average note rate	4.09%	4.18%
Weighted-average age (in years)	3 years 6 months	3 years 2 months 12 days
Weighted-average expected prepayment (constant prepayment rate)	15.10%	14.80%
Weighted-average expected life (in years)	5 years 4 months 24 days	5 years 6 months
Weighted-average discount rate	11.20%	11.20%
Conventional [Member] | Mortgage Servicing Rights [Member]
Servicing Assets at Fair Value [Line Items]
Servicing portfolio	$ 162,730	$ 162,620
Fair value	$ 1,699	$ 1,699
Value (bps) | BasisPoint	104	104
Weighted-average servicing fees (bps) | BasisPoint	27	27
Multiple (value/servicing fees) | Multiple	3.85	3.85
Weighted-average note rate	4.09%	4.14%
Weighted-average age (in years)	3 years 4 months 24 days	3 years 1 month 6 days
Weighted-average expected prepayment (constant prepayment rate)	11.40%	11.40%
Weighted-average expected life (in years)	6 years 3 months 18 days	6 years 6 months
Weighted-average discount rate	9.50%	9.60%