Mortgage Servicing Assets - Schedule of Range and Weighted-Average of Significant Unobservable Inputs (Detail) - Discounted Cash Flow [Member] - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
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	2.38%	0.80%
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%	15.00%
Escrow earn rate	3.94%	3.00%
Servicing cost	$ 38,500,000,000	$ 38,500,000,000
Loan assumption rate	3.22%	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	1.16%	1.20%
Residual cash flows discount rate	9.10%	8.90%
Escrow earn rate	3.07%	2.40%
Servicing cost	$ 1,445,000,000	$ 1,468,000,000
Loan assumption rate	1.34%	1.24%
Percentage late	0.20%	0.27%
Residential Mortgage Backed Securities [Member] | Minimum [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults	7.50%	8.50%
Servicing cost	$ 62,000,000	$ 76,000,000
Prepayment speed	8.27%	7.95%
Residential Mortgage Backed Securities [Member] | Maximum [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults	10.00%	11.00%
Servicing cost	$ 4,375,000,000	$ 3,335,000,000
Prepayment speed	55.15%	32.49%
Residential Mortgage Backed Securities [Member] | Weighted Average [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults	7.54%	8.55%
Servicing cost	$ 67,760,000	$ 82,190,000
Prepayment speed	9.35%	10.15%