Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Hybrid Securities [Line Items]
Hybrid securities	$ 159.9	$ 219.5	$ 229.6
Fixed maturities
Hybrid Securities [Line Items]
Hybrid securities	159.9	219.5	216.0
Fixed maturities | State and local government obligations
Hybrid Securities [Line Items]
Hybrid securities	2.4	0.0	0.0
Fixed maturities | Corporate debt securities
Hybrid Securities [Line Items]
Hybrid securities	114.9	40.1	31.2
Fixed maturities | Residential mortgage-backed securities
Hybrid Securities [Line Items]
Hybrid securities	0.0	170.5	175.3
Fixed maturities | Other asset-backed securities
Hybrid Securities [Line Items]
Hybrid securities	7.1	8.9	9.5
Fixed maturities | Redeemable preferred stocks
Hybrid Securities [Line Items]
Hybrid securities	35.5	0.0	0.0
Equity securities | Nonredeemable preferred stocks
Hybrid Securities [Line Items]
Hybrid securities	$ 0.0	$ 0.0	$ 13.6