Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unrealized gains and losses on securities:
Net of taxes	$ 4.9	$ (28.2)
Unrealized Gains and Losses on Securities [Member]
Unrealized gains and losses on securities:
Net realized investment losses (gains)	5.4	(41.6)	$ (23.2)
Provision (benefit) for income taxes	(0.5)	13.4	13.2
Net of taxes	$ 4.9	$ (28.2)	$ (10.0)