Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Investments [Line Items]
Gross realized investment and other gains	$ 54.4	$ 74.2	$ 47.3
Gross realized investment and other losses	(28.7)	(25.0)	(10.5)
Net realized investment and other gains (losses)	25.7	49.2	36.8
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	27.2	30.5	31.5
Gross realized investment and other losses	(9.1)	(11.6)	(8.2)
Equity Securities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	0.4	26.6	6.3
Gross realized investment and other losses	(0.5)	(0.7)	(1.1)
Other Investments [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	21.5	16.1	8.9
Gross realized investment and other losses	(15.1)	(10.5)	(0.3)
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	0.5	1.0	0.2
Gross realized investment and other losses	(0.3)	(1.0)	(0.1)
Gain On Sale Of Affiliate [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	4.8	0	0
Gain On Sale Of Real Estate [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	0	0	0.4
Other-Than-Temporary Impairment Losses On Fixed Maturities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other losses	(1.5)	(0.1)	(0.8)
Other-Than-Temporary Impairment Losses On Equity Securities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other losses	$ (2.2)	$ (1.1)	$ 0