Investments - Schedule of Investment Income and Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment income:
Interest on fixed maturities	$ 115.0	$ 97.1	$ 88.9
Dividends on equity securities	12.5	13.9	15.6
Income on alternative investments	19.8	49.5	29.9
Income on short-term and other investments	9.5	7.7	0.4
Investment income	156.8	168.2	134.8
Investment expenses	(23.7)	(28.2)	(19.7)
Net investment income	$ 133.1	$ 140.0	$ 115.1