Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Lease expiration date	Jan. 01, 2030
Minimum sublease rental income	$ 12.9
Letters of credit outstanding	69.4
Accrued loan loss	32.9	$ 28.2
Assets available for recourse	614.5
Warehouse receivables	$ 370.9
Letters of credit expiration date	2018-09
Guarantees total	$ 56.1
Commitments to investment in future real estate investment	38.6
Commitments to investment in unconsolidated real estate subsidiary	$ 20.8
Maximum [Member]
Loss Contingencies [Line Items]
Co-investments typically range	2.00%
Funded loans subject to loss sharing arrangements [Member]
Loss Contingencies [Line Items]
Funded loans unpaid principal	$ 19,800.0
Letters of credit outstanding	58.0	$ 45.0
SBL Program [Member]
Loss Contingencies [Line Items]
Letters of credit outstanding	5.0
Funded loans not subject to loss sharing arrangements [Member]
Loss Contingencies [Line Items]
Letters of credit outstanding	$ 63.0