Fair Value Summary of Changes in Fair Value Associated With Level 3 Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	$ 71.1		$ 158.7		$ 68.7		$ 114.7
Calls/ Maturities/ Paydowns	(1.2)		(6.8)		(1.8)		(12.5)
Purchases	0		80.2		0		125.4
Sales	(0.1)		0		(0.1)		0
Net Realized (Gain) Loss on Sales	0.1		(0.5)		0.1		(0.5)
Change in Valuation	2.0		26.6		5.0		31.1
Net Transfers in (out)	0		0		0		0
Ending Fair value	71.9		258.2		71.9		258.2
Fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	28.6		112.9		29.2		70.8
Calls/ Maturities/ Paydowns	(1.3)		(6.7)		(1.8)		(12.4)
Purchases	0		79.9		0		125.1
Sales	(0.1)		0		(0.1)		0
Net Realized (Gain) Loss on Sales	0.1		0		0.1		0
Change in Valuation	0.3		(1.4)		0.2		1.2
Net Transfers in (out)	0		0		0		0
Ending Fair value	27.6		184.7		27.6		184.7
Fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0		85.6		0.2		45.5
Calls/ Maturities/ Paydowns	0		(6.3)		0		(11.6)
Purchases	0		79.9		0		125.1
Sales	(0.1)		0		(0.1)		0
Net Realized (Gain) Loss on Sales	0.1		0		0.1		0
Change in Valuation	0		(2.1)		(0.2)		(1.9)
Net Transfers in (out)	0		0		0		0
Ending Fair value	0		157.1		0		157.1
Fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	28.6		27.3		29.0		25.3
Calls/ Maturities/ Paydowns	(1.3)		(0.4)		(1.8)		(0.8)
Purchases	0		0		0		0
Sales	0		0		0		0
Net Realized (Gain) Loss on Sales	0		0		0		0
Change in Valuation	0.3		0.7		0.4		3.1
Net Transfers in (out)	0		0		0		0
Ending Fair value	27.6		27.6		27.6		27.6
Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	42.1	[1]	33.8	[2]	39.0	[3]	31.9	[4]
Calls/ Maturities/ Paydowns	0	[1]	0	[2]	0	[3]	0	[4]
Purchases	0	[1]	0	[2]	0	[3]	0	[4]
Sales	0	[1]	0	[2]	0	[3]	0	[4]
Net Realized (Gain) Loss on Sales	0	[1]	0	[2]	0	[3]	0	[4]
Change in Valuation	1.7	[1]	1.0	[2]	4.8	[3]	2.9	[4]
Net Transfers in (out)	0	[1]	0	[2]	0	[3]	0	[4]
Ending Fair value	43.8	[1],[3]	34.8	[2],[4]	43.8	[1],[3]	34.8	[2],[4]
Equity securities | Common equities | Other risk investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0.4		12.0		0.5		12.0
Calls/ Maturities/ Paydowns	0.1		(0.1)		0		(0.1)
Purchases	0		0.3		0		0.3
Sales	0		0		0		0
Net Realized (Gain) Loss on Sales	0		(0.5)		0		(0.5)
Change in Valuation	0		27.0		0		27.0
Net Transfers in (out)	0		0		0		0
Ending Fair value	$ 0.5		$ 38.7		$ 0.5		$ 38.7

[1] The $1.7 million represents net holding period gains on a hybrid security, which is reflected in net realized gains (losses) on securities in the comprehensive income statement.

[2] The $1.0 million represents net holding period gains on a hybrid security, which is reflected in net realized gains (losses) on securities in the comprehensive income statement.

[3] The $4.8 million represents net holding period gains on a hybrid security, which is reflected in net realized gains (losses) on securities in the comprehensive income statement.

[4] The $2.9 million represents net holding period gains on a hybrid security, which is reflected in net realized gains (losses) on securities in the comprehensive income statement.