Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Hybrid Securities [Line Items]
Hybrid securities	$ 319.5	$ 427.7	$ 255.1
Fixed maturities
Hybrid Securities [Line Items]
Hybrid securities	252.9	305.4	189.0
Fixed maturities | State and local government obligations
Hybrid Securities [Line Items]
Hybrid securities	0.0	0.0	5.1
Fixed maturities | Corporate debt securities
Hybrid Securities [Line Items]
Hybrid securities	105.6	139.8	142.0
Fixed maturities | Residential mortgage-backed securities
Hybrid Securities [Line Items]
Hybrid securities	117.5	120.7	27.6
Fixed maturities | Commercial mortgage-backed securities
Hybrid Securities [Line Items]
Hybrid securities	17.3	31.2	0.0
Fixed maturities | Other asset-backed securities
Hybrid Securities [Line Items]
Hybrid securities	12.5	13.7	14.3
Equity securities | Nonredeemable preferred stocks
Hybrid Securities [Line Items]
Hybrid securities	$ 66.6	$ 122.3	$ 66.1