Mortgage Servicing Assets - Schedule of Range and Weighted-Average of Significant Unobservable Inputs (Details) - Discounted cash flow - USD ($)	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Real estate — commercial mortgage | Minimum
Servicing Assets at Fair Value [Line Items]
Expected defaults	1.00%	1.00%
Residual cash flows discount rate	6.97%	7.00%
Escrow earn rate	0.78%	2.22%
Loan assumption rate	0.00%	0.00%
Real estate — commercial mortgage | Maximum
Servicing Assets at Fair Value [Line Items]
Expected defaults	2.00%	2.00%
Residual cash flows discount rate	11.42%	15.00%
Escrow earn rate	1.92%	3.70%
Loan assumption rate	3.37%	3.18%
Real estate — commercial mortgage | Weighted Average
Servicing Assets at Fair Value [Line Items]
Expected defaults	1.15%	1.14%
Residual cash flows discount rate	9.42%	9.19%
Escrow earn rate	1.66%	2.98%
Loan assumption rate	1.32%	1.39%
Residential mortgage-backed securities | Minimum
Servicing Assets at Fair Value [Line Items]
Expected defaults	7.55%	7.50%
Prepayment speed	12.29%	9.32%
Servicing cost	$ 62,000,000	$ 62,000,000
Residential mortgage-backed securities | Maximum
Servicing Assets at Fair Value [Line Items]
Expected defaults	9.27%	10.00%
Prepayment speed	55.18%	58.76%
Servicing cost	$ 5,135,000,000	$ 4,375,000,000
Residential mortgage-backed securities | Weighted Average
Servicing Assets at Fair Value [Line Items]
Expected defaults	7.64%	7.54%
Prepayment speed	16.89%	9.93%
Servicing cost	$ 77,450,000	$ 68,230,000