Investments - Schedule of Recognized Other-than-temporary Losses on Fixed Maturities and Equity Portfolios (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other-than-temporary impairment:
Other-than-temporary impairment losses	$ (20.3)	$ (7.6)	$ (2.5)
Equity Securities [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	0.0	0.0	(1.7)
Other Invested Assets [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	0.0	(1.0)	0.0
Foreign Governments [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	(2.0)	0.0	0.0
Obligations of States and Political Subdivisions [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	(0.1)	0.0	(0.1)
Corporate Bonds [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	$ (18.2)	$ (6.6)	$ (0.7)