Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	$ (21.6)	$ 24.8	$ (58.8)	$ 42.7
Income tax benefit (expense)	2.8	(5.1)	9.1	(9.7)
Net unrealized investment (losses) gains, net of income taxes	(18.8)	19.7	(49.7)	33.0
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	(21.8)	19.5	(58.9)	32.0
Equity Securities [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	0.0	3.3	0.0	8.7
Other [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	0.1	2.0	0.1	2.0
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	$ 0.1	$ 0.0	$ 0.0	$ 0.0