Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Schedule Of Investments [Line Items]
Gross realized investment gains and other	$ 29.1	$ 23.8
Gross realized investment and other losses	(13.1)	(12.7)
Net realized investment and other gains	16.0	11.1
Other Investments [Member]
Schedule Of Investments [Line Items]
Gross realized investment gains and other	20.9	12.7
Gross realized investment and other losses	(7.1)	(4.9)
Other Assets [Member]
Schedule Of Investments [Line Items]
Gross realized investment and other losses	0	(4.0)
Fixed Maturities [Member]
Schedule Of Investments [Line Items]
Gross realized investment gains and other	4.1	4.6
Gross realized investment and other losses	(3.7)	(3.5)
Equity Securities [Member]
Schedule Of Investments [Line Items]
Gross realized investment gains and other	3.8	6.5
Gross realized investment and other losses	(0.7)	(0.1)
Real Estate Holdings [Member]
Schedule Of Investments [Line Items]
Gross realized investment gains and other	0.3	0
Short-Term Investments [Member]
Schedule Of Investments [Line Items]
Gross realized investment and other losses	(1.1)	(0.2)
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule Of Investments [Line Items]
Gross realized investment and other losses	(0.4)	0
Other-Than-Temporary Impairment Losses on Equity Securities [Member]
Schedule Of Investments [Line Items]
Gross realized investment and other losses	$ (0.1)	$ 0