Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Investments [Line Items]
Net unrealized investment and other gains before income taxes	$ 21.1	$ 2.8	$ 63.8	$ 72.6
Income tax provision	(4.6)	0.3	(14.3)	(15.7)
Net unrealized investment and other gains, net of income taxes	16.5	3.1	49.5	56.9
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Net unrealized investment and other gains before income taxes	9.5	13.7	41.6	78.5
Equity Securities [Member]
Schedule of Investments [Line Items]
Net unrealized investment and other gains before income taxes	11.7	(10.8)	20.2	(6.8)
Other Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment and other gains before income taxes	(0.1)	0.0	2.0	0.6
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment and other gains before income taxes	$ 0.0	$ (0.1)	$ 0.0	$ 0.3