Summary of Changes in Fair Value Associated With Level 3 Assets (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net realized gains (losses) on securities	$ 29.8	$ 172.3	$ 245.1	$ 251.4
Equity securities | Nonredeemable preferred stocks | Hybrid Securities | Financials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net realized gains (losses) on securities	1.3	0.8	4.2	0.8
Asset-backed Securities | Fixed maturities | Residential mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfer out of Level 3 and into Level 2	$ (141.4)		$ (141.4)