Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (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]
Gross realized investment and other gains	$ 22.9	$ 31.6	$ 74.5	$ 94.6
Gross realized investment and other losses	(16.9)	(13.9)	(49.4)	(81.8)
Net realized investment and other gains	6.0	17.7	25.1	12.8
Income tax expense	(2.2)	(6.5)	(7.9)	(7.7)
Net realized investment and other gains (losses), net of income taxes	3.8	11.2	17.2	5.1
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	6.3	5.4	18.9	16.4
Gross realized investment and other losses	(4.8)	(6.1)	(16.0)	(21.0)
Equity Securities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	9.4	20.8	37.1	45.4
Gross realized investment and other losses	(1.4)	(1.9)	(3.9)	(9.0)
Other Investments [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	7.0	4.2	17.8	31.2
Gross realized investment and other losses	(10.5)	(3.5)	(27.7)	(42.7)
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	0.2	0.0	0.7	0.4
Gross realized investment and other losses	0.0	0.0	(0.1)	(0.1)
Other Assets [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	0.0	1.2	0.0	1.2
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other losses	(0.1)	(0.6)	(0.1)	(1.7)
Other-Than-Temporary Impairment Losses on Equity Securities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other losses	$ (0.1)	$ (1.8)	$ (1.6)	$ (7.3)