Reinsurance - Premium Income Data (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Disclosures [Abstract]
Direct Premium Income									$ 9,171.1	$ 8,677.2	$ 8,385.0
Reinsurance Assumed									142.6	230.0	236.3
Reinsurance Ceded									(327.6)	(310.1)	(263.6)
Premium Income	$ 2,263.3	$ 2,251.8	$ 2,221.0	$ 2,250.0	$ 2,158.4	$ 2,153.6	$ 2,142.2	$ 2,142.9	8,986.1	8,597.1	8,357.7
Ceded Benefits and Change in Reserves for Future Benefits									$ 667.2	$ 655.4	$ 648.3