Derivatives - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Option Indexed to Issuer's Equity [Line Items]
Replacement cost of contracts in a gain position	$ 51.6		$ 51.6		$ 12.7
Change in fair value of rate lock commitments, net	(7.8)	$ (2.2)	7.3	$ (0.1)
Rate Lock Commitments [Member]
Option Indexed to Issuer's Equity [Line Items]
Change in fair value of rate lock commitments, net	$ 0.8	$ 1.0	$ 3.2	$ 1.8