Mortgage Servicing Rights, Net - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Mortgage Servicing Rights
Primary servicing portfolio	$ 64,600.0	$ 59,900.0
Special servicing portfolio	2,300.0	2,400.0
Discount Rate One
Mortgage Servicing Rights
Decrease in fair value of servicing rights	13.2	11.9
Discount Rate Two
Mortgage Servicing Rights
Decrease in fair value of servicing rights	$ 25.7	$ 23.3
Minimum
Mortgage Servicing Rights
Discount rates used in measuring fair value (in percentage)	6.10%	6.10%
Minimum | Discount Rate One
Mortgage Servicing Rights
Increase in discount rate (in percentage)	1.00%	1.00%
Maximum
Mortgage Servicing Rights
Discount rates used in measuring fair value (in percentage)	13.50%	13.50%
Maximum | Discount Rate Two
Mortgage Servicing Rights
Increase in discount rate (in percentage)	2.00%	2.00%
Mortgage Servicing Rights
Mortgage Servicing Rights
Estimated fair value of MSRs	$ 482.4	$ 441.7