Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	$ (126.2)	$ 59.9
Income tax benefit (provision)	19.2	(9.6)
Net unrealized investment (losses) gains, net of income taxes	(107.0)	50.3
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	(111.8)	59.9
Other Investment [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	(14.2)	0.0
Short-term Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	$ (0.2)	$ 0.0