Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (4.6)	$ (4.8)	$ 99.0	$ (63.6)
Income tax (provision) benefit	0.4	1.4	(16.1)	10.5
Net unrealized investment gains (losses), net of income taxes	(4.2)	(3.4)	82.9	(53.1)
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	(3.8)	(4.3)	95.3	(63.2)
Other Investment [Member]
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	0.0	0.0	4.4		$ 0.1
Short-term Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (0.8)	$ (0.5)	$ (0.7)	$ (0.5)