Mortgage Servicing Assets - Schedule of Range and Weighted-Average of Significant Unobservable Inputs (Detail) (Mortgage Servicing Assets [Member], Discounted Cash Flow [Member], USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Servicing Assets at Fair Value [Line Items]
Prepayment speed	6.20%	10.40%
Expected defaults	2.10%	2.40%
Residual cash flows discount rate	8.00%	9.20%
Value assigned to escrow funds	1.90%	1.50%
Servicing cost	$ 1,039	$ 2,582
Loan assumption rate	1.54%	2.24%
Percentage late	0.33%	0.22%
Minimum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	0.00%	0.00%
Expected defaults	1.10%	1.00%
Residual cash flows discount rate	7.00%	7.00%
Value assigned to escrow funds	0.35%	0.27%
Servicing cost	150	950
Loan assumption rate	0.00%	0.00%
Percentage late	0.00%	0.00%
Maximum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	25.00%	25.00%
Expected defaults	3.00%	3.00%
Residual cash flows discount rate	15.00%	15.00%
Value assigned to escrow funds	3.58%	2.78%
Servicing cost	$ 9,296	$ 20,200
Loan assumption rate	3.00%	3.00%
Percentage late	2.00%	2.00%