Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	$ (4.8)	$ 21.1	$ (63.6)	$ 63.8
Income tax benefit (expense)	1.4	(4.6)	10.5	(14.3)
Net unrealized investment (losses) gains, net of income taxes	(3.4)	16.5	(53.1)	49.5
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	(4.3)	9.5	(63.2)	41.6
Equity Securities [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	0.0	11.7	0.0	20.2
Other [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	0.0	(0.1)	0.1	2.0
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	$ (0.5)	$ 0.0	$ (0.5)	$ 0.0