Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 97.9	$ (88.5)	$ 64.1
Income tax (provision) benefit	(15.4)	13.0	(14.6)
Net unrealized investment gains (losses), net of income taxes	82.5	(75.5)	49.5
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	93.3	(88.1)	25.4
Equity Securities [Member]
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	0.0	0.0	36.6
Other Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	4.4	0.1	2.1
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 0.2	$ (0.5)	$ 0.0