Mortgage Servicing Assets - Schedule of Range and Weighted-Average of Significant Unobservable Inputs (Detail) - Discounted Cash Flow [Member] - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Weighted Average [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	4.90%	5.90%
Expected defaults	1.70%	1.90%
Residual cash flows discount rate	7.80%	7.80%
Escrow earn rate	2.30%	1.70%
Servicing cost	$ 1,151,000,000	$ 1,053,000,000
Loan assumption rate	1.37%	1.52%
Percentage late	0.33%	0.32%
Minimum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	1.70%	1.90%
Expected defaults	1.00%	1.00%
Residual cash flows discount rate	7.00%	7.00%
Escrow earn rate	0.90%	0.50%
Servicing cost	$ 150,000,000	$ 150,000,000
Loan assumption rate	0.00%	0.20%
Percentage late	0.00%	0.00%
Maximum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	16.30%	12.30%
Expected defaults	3.00%	3.00%
Residual cash flows discount rate	15.00%	14.10%
Escrow earn rate	3.50%	3.10%
Servicing cost	$ 2,719,000,000	$ 2,700,000,000
Loan assumption rate	3.00%	3.00%
Percentage late	2.00%	2.00%