Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Hybrid Securities [Line Items]
Hybrid securities	$ 229.6	$ 272.7	$ 290.8
Fixed maturities
Hybrid Securities [Line Items]
Hybrid securities	216.0	222.0	236.7
Fixed maturities | Corporate debt securities
Hybrid Securities [Line Items]
Hybrid securities	31.2	49.1	89.4
Fixed maturities | Residential mortgage-backed securities
Hybrid Securities [Line Items]
Hybrid securities	175.3	144.3	117.7
Fixed maturities | Commercial mortgage-backed securities
Hybrid Securities [Line Items]
Hybrid securities	0.0	17.3	17.7
Fixed maturities | Other asset-backed securities
Hybrid Securities [Line Items]
Hybrid securities	9.5	11.3	11.9
Equity securities | Nonredeemable preferred stocks
Hybrid Securities [Line Items]
Hybrid securities	$ 13.6	$ 50.7	$ 54.1