Mortgage Servicing Assets - Schedule of Range and Weighted-Average of Significant Unobservable Inputs (Detail) - Discounted Cash Flow [Member] - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Commercial Mortgage Backed Securities [Member] | Minimum [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults	1.00%	1.00%
Residual cash flows discount rate	7.00%	7.00%
Escrow earn rate	1.50%	1.10%
Servicing cost	$ 150,000,000	$ 150,000,000
Loan assumption rate	0.00%	0.00%
Percentage late	0.00%	0.00%
Commercial Mortgage Backed Securities [Member] | Maximum [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults	3.00%	3.00%
Residual cash flows discount rate	15.00%	12.00%
Escrow earn rate	3.10%	3.00%
Servicing cost	$ 38,500,000,000	$ 2,700,000,000
Loan assumption rate	3.00%	3.00%
Percentage late	2.00%	2.00%
Commercial Mortgage Backed Securities [Member] | Weighted Average [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults	1.30%	1.40%
Residual cash flows discount rate	8.80%	8.00%
Escrow earn rate	2.50%	2.40%
Servicing cost	$ 1,457,000,000	$ 1,124,000,000
Loan assumption rate	1.24%	1.13%
Percentage late	0.29%	0.34%
Residential Mortgage Backed Securities [Member] | Minimum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	7.85%
Expected defaults	8.50%
Servicing cost	$ 76,000,000
Residential Mortgage Backed Securities [Member] | Maximum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	21.72%
Expected defaults	11.00%
Servicing cost	$ 3,335,000,000
Residential Mortgage Backed Securities [Member] | Weighted Average [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	9.42%
Expected defaults	8.55%
Servicing cost	$ 82,180,000