Reinsurance - Premium Income Data (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reinsurance Disclosures [Abstract]
Direct Premium Income									$ 8,151.9	$ 7,899.3	$ 7,777.3
Reinsurance Assumed									202.8	189.8	203.2
Reinsurance Ceded									(272.3)	(291.9)	(355.8)
Premium Income	$ 2,037.8	$ 2,020.8	$ 2,017.5	$ 2,006.3	$ 1,967.9	$ 1,947.2	$ 1,943.6	$ 1,938.5	8,082.4	7,797.2	7,624.7
Ceded Benefits and Change in Reserves for Future Benefits									$ 662.7	$ 662.7	$ 728.7