Mortgage Servicing Rights, Net - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Mortgage Servicing Rights [Line Items]
Primary servicing portfolio	$ 59,900.0	$ 57,100.0
Special servicing portfolio	2,400.0	2,900.0
Discount Rate One
Mortgage Servicing Rights [Line Items]
Decrease in fair value of servicing rights	11.9	12.4
Discount Rate Two
Mortgage Servicing Rights [Line Items]
Decrease in fair value of servicing rights	$ 23.3	$ 24.4
Minimum
Mortgage Servicing Rights [Line Items]
Discount rates used in measuring fair value	6.10%	3.00%
Minimum | Discount Rate One
Mortgage Servicing Rights [Line Items]
Increase in discount rate	1.00%	1.00%
Maximum
Mortgage Servicing Rights [Line Items]
Discount rates used in measuring fair value	13.50%	13.50%
Maximum | Discount Rate Two
Mortgage Servicing Rights [Line Items]
Increase in discount rate	2.00%	2.00%
Mortgage Servicing Rights
Mortgage Servicing Rights [Line Items]
Estimated fair value of MSRs	$ 441,700.0	$ 451.9