Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Investments [Line Items]
Net unrealized investment gains before income taxes	$ 136.4	$ 43.7	$ 10.2	$ 103.6
Income tax provision	24.4	6.9	5.2	16.5
Net unrealized investment gains, net of income taxes	112.0	36.8	5.0	87.1
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains before income taxes	136.2	39.2	24.4	99.1
Other investments
Schedule of Investments [Line Items]
Net unrealized investment gains before income taxes	0.0	0.0	(14.2)	0.0
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains before income taxes	$ 0.2	$ 4.5	$ 0.0	$ 4.5