Mortgage Servicing Rights, Net - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Mortgage Servicing Rights [Line Items]
Primary servicing portfolio	$ 55,100.0	$ 54,200.0
Special servicing portfolio	$ 3,600.0	$ 3,800.0
Minimum [Member]
Mortgage Servicing Rights [Line Items]
Discount rate	3.00%	3.00%
Maximum [Member]
Mortgage Servicing Rights [Line Items]
Discount rate	13.50%	13.50%
Discount Rate One [Member]
Mortgage Servicing Rights [Line Items]
Increase in discount rate	1.00%	1.00%
Decrease in fair value of servicing rights	$ (13.7)	$ (11.8)
Discount Rate Two [Member]
Mortgage Servicing Rights [Line Items]
Increase in discount rate	2.00%	2.00%
Decrease in fair value of servicing rights	$ (25.0)	$ (23.0)
Mortgage Servicing Rights [Member]
Mortgage Servicing Rights [Line Items]
Estimated fair value of MSRs	$ 422.2	$ 418.1