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	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Servicing Assets at Fair Value [Line Items]
Prepayment speed, rate	8.00%	11.70%
Expected defaults, rate	2.30%	2.40%
Residual cash flows discount rate	8.90%	9.40%
Value assigned to escrow funds	1.50%	1.80%
Servicing cost	$ 5,425	$ 2,512
Loan assumption rate	2.38%	2.18%
Percentage late	0.21%	0.22%
Minimum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed, rate	0.00%	0.00%
Expected defaults, rate	1.10%	1.00%
Residual cash flows discount rate	7.00%	7.00%
Value assigned to escrow funds	0.25%	0.50%
Servicing cost	150	700
Loan assumption rate	0.00%	0.00%
Percentage late	0.00%	0.00%
Maximum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed, rate	25.00%	25.00%
Expected defaults, rate	3.00%	3.00%
Residual cash flows discount rate	15.00%	15.00%
Value assigned to escrow funds	2.75%	3.75%
Servicing cost	$ 23,018	$ 17,000
Loan assumption rate	3.00%	3.00%
Percentage late	2.00%	2.00%