Deferred Acquisition Costs Deferred Acquisition Costs (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Deferred Policy Acquisition Cost, Period Start	$ 111.1	$ 101.9
Deferred Policy Acquisition Cost, Capitalization	91.1	84.0
Deferred Policy Acquisition Costs, Disposition	(88.9)	(81.6)
Deferred Policy Acquisition Cost, Period End	$ 113.3	$ 104.3