Reinsurance - Premium Income Data (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Direct Premium Income									$ 8,385.0	$ 8,151.9	$ 7,899.3
Reinsurance Assumed									236.3	202.8	189.8
Reinsurance Ceded									(263.6)	(272.3)	(291.9)
Premium Income	$ 2,099.2	$ 2,089.4	$ 2,081.6	$ 2,087.5	$ 2,037.8	$ 2,020.8	$ 2,017.5	$ 2,006.3	8,357.7	8,082.4	7,797.2
Ceded Benefits and Change in Reserves for Future Benefits									$ 648.3	$ 662.7	$ 662.7