Derivatives - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Option Indexed to Issuer's Equity [Line Items]
Replacement cost of contracts in a gain position	$ 94.7		$ 94.7		$ 12.7
Change in fair value of rate lock commitments, net	(6.8)	$ 0.6	0.5	$ 0.5
Rate Lock Commitments [Member]
Option Indexed to Issuer's Equity [Line Items]
Change in fair value of rate lock commitments, net	$ 1.3	$ (0.1)	$ (2.0)	$ (1.8)