Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 43.7	$ (21.6)	$ 103.6	$ (58.8)
Income tax (provision) benefit	(6.9)	2.8	(16.5)	9.1
Net unrealized investment gains (losses), net of income taxes	36.8	(18.8)	87.1	(49.7)
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	39.2	(21.8)	99.1	(58.9)
Other and Short-Term Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 4.5	$ 0.2	$ 4.5	$ 0.1