Investments - Schedule of Investment Income and Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investment income:
Interest on fixed maturities	$ 129.5	$ 115.0	$ 97.1
Dividends on equity securities	11.1	12.5	13.9
Income on alternative investments	22.4	19.8	49.5
Income on short-term and other investments	8.9	9.5	7.7
Investment income	171.9	156.8	168.2
Investment expenses	(20.8)	(23.7)	(28.2)
Net investment income	$ 151.1	$ 133.1	$ 140.0