Reinsurance - Premium Income Data (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effects of Reinsurance
Direct Premium Income									$ 9,621.9	$ 9,576.3	$ 9,171.1
Reinsurance Assumed									94.1	116.5	142.6
Reinsurance Ceded									(337.9)	(327.2)	(327.6)
Premium Income	$ 2,319.9	$ 2,318.1	$ 2,368.7	$ 2,371.4	$ 2,352.6	$ 2,331.2	$ 2,343.1	$ 2,338.7	9,378.1	9,365.6	8,986.1
Ceded Benefits and Change in Reserves for Future Benefits									$ 628.8	$ 650.1	$ 667.2