Investments - Schedule of Investment Income and Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Investment income:
Interest on fixed maturities	$ 98.9	$ 108.6	$ 129.5
Dividends on equity securities	3.1	2.8	11.1
Income on alternative investments	95.5	10.4	22.4
Income on short-term and other investments	3.3	3.9	8.9
Investment income	200.8	125.7	171.9
Investment expenses	(13.2)	(13.0)	(20.8)
Net investment income	$ 187.6	$ 112.7	$ 151.1