Reinsurance - Premium Income Data (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Disclosures [Abstract]
Direct Premium Income									$ 8,677.2	$ 8,385.0	$ 8,151.9
Reinsurance Assumed									230.0	236.3	202.8
Reinsurance Ceded									(310.1)	(263.6)	(272.3)
Premium Income	$ 2,158.4	$ 2,153.6	$ 2,142.2	$ 2,142.9	$ 2,099.2	$ 2,089.4	$ 2,081.6	$ 2,087.5	8,597.1	8,357.7	8,082.4
Ceded Benefits and Change in Reserves for Future Benefits									$ 655.4	$ 648.3	$ 662.7