Hybrid Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Hybrid Securities [Line Items]
Hybrid securities	$ 244.4	$ 245.3	$ 244.4
Fixed maturities
Hybrid Securities [Line Items]
Hybrid securities	187.1	192.5	193.9
Fixed maturities | Corporate debt securities
Hybrid Securities [Line Items]
Hybrid securities	171.8	176.1	177.5
Fixed maturities | Other asset-backed securities
Hybrid Securities [Line Items]
Hybrid securities	15.3	16.4	16.4
Equity securities | Nonredeemable preferred stocks
Hybrid Securities [Line Items]
Hybrid securities	$ 57.3	$ 52.8	$ 50.5