Fair Value Summary of Changes in Fair Value Associated With Level 3 Assets (Detail) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2015		Jun. 30, 2014		Jun. 30, 2015		Jun. 30, 2014		Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total investments at fair value	[1],[2]	$ 20,577.4		$ 18,759.2		$ 20,577.4		$ 18,759.2		$ 19,018.0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value		81.6		71.1		81.3		68.7
Calls/ Maturities/ Paydowns		(0.4)		(1.2)		(0.6)		(1.8)
Purchases		1.0		0.0		1.0		0.0
Sales		0.0		(0.1)		0.0		(0.1)
Net Realized (Gain) Loss on Sales		(39.4)		0.1		(39.4)		0.1
Change in Valuation		(2.1)		2.0		(1.6)		5.0
Net Transfers in (out)		(28.5)		0.0		(28.5)		0.0
Ending Fair value		12.2		71.9		12.2		71.9
Fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value		11.4		28.6		11.6		29.2
Calls/ Maturities/ Paydowns		(0.4)		(1.3)		(0.6)		(1.8)
Purchases		1.0		0.0		1.0		0.0
Sales		0.0		(0.1)		0.0		(0.1)
Net Realized (Gain) Loss on Sales		0.0		0.1		0.0		0.1
Change in Valuation		(0.1)		0.3		(0.1)		0.2
Net Transfers in (out)		0.0		0.0		0.0		0.0
Ending Fair value		11.9		27.6		11.9		27.6
Fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value		0.0		0.0		0.0		0.0
Calls/ Maturities/ Paydowns		0.0		0.0		0.0		0.0
Purchases		1.0		0.0		1.0		0.0
Sales		0.0		0.0		0.0		0.0
Net Realized (Gain) Loss on Sales		0.0		0.0		0.0		0.0
Change in Valuation		0.0		0.0		0.0		0.0
Net Transfers in (out)		0.0		0.0		0.0		0.0
Ending Fair value		1.0		0.0		1.0		0.0
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		0.0		0.0		0.0		0.2
Calls/ Maturities/ Paydowns		0.0		0.0		0.0		0.0
Purchases		0.0		0.0		0.0		0.0
Sales		0.0		(0.1)		0.0		(0.1)
Net Realized (Gain) Loss on Sales		0.0		0.1		0.0		0.1
Change in Valuation		0.0		0.0		0.0		(0.2)
Net Transfers in (out)		0.0		0.0		0.0		0.0
Ending Fair value		0.0		0.0		0.0		0.0
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		11.4		28.6		11.6		29.0
Calls/ Maturities/ Paydowns		(0.4)		(1.3)		(0.6)		(1.8)
Purchases		0.0		0.0		0.0		0.0
Sales		0.0		0.0		0.0		0.0
Net Realized (Gain) Loss on Sales		0.0		0.0		0.0		0.0
Change in Valuation		(0.1)		0.3		(0.1)		0.4
Net Transfers in (out)		0.0		0.0		0.0		0.0
Ending Fair value		10.9		27.6		10.9		27.6
Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value		69.9	[3]	42.1	[4]	69.3	[3]	39.0	[4]
Calls/ Maturities/ Paydowns		0.0	[3]	0.0	[4]	0.0	[3]	0.0	[4]
Purchases		0.0	[3]	0.0	[4]	0.0	[3]	0.0	[4]
Sales		0.0	[3]	0.0	[4]	0.0	[3]	0.0	[4]
Net Realized (Gain) Loss on Sales		(39.4)	[3]	0.0	[4]	(39.4)	[3]	0.0	[4]
Change in Valuation		(2.0)	[3]	1.7	[4]	(1.4)	[3]	4.8	[4]
Net Transfers in (out)		(28.5)	[3]	0.0	[4]	(28.5)	[3]	0.0	[4]
Ending Fair value		0.0	[3]	43.8	[4]	0.0	[3]	43.8	[4]
Equity securities | Common equities | Other risk investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value		0.3		0.4		0.4		0.5
Calls/ Maturities/ Paydowns		0.0		0.1		0.0		0.0
Purchases		0.0		0.0		0.0		0.0
Sales		0.0		0.0		0.0		0.0
Net Realized (Gain) Loss on Sales		0.0		0.0		0.0		0.0
Change in Valuation		0.0		0.0		(0.1)		0.0
Net Transfers in (out)		0.0		0.0		0.0		0.0
Ending Fair value		0.3		0.5		0.3		0.5
Fair Value, Inputs, Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total investments at fair value		6,230.2		8,752.6		6,230.2		8,752.6		7,314.3
Fair Value, Inputs, Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total investments at fair value		14,335.0		9,934.7		14,335.0		9,934.7		11,622.4
Fair Value, Inputs, Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total investments at fair value		12.2		71.9		12.2		71.9		81.3
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total investments at fair value		$ 19,174.0		$ 17,146.4		$ 19,174.0		$ 17,146.4		$ 17,402.8

[1] Our portfolio reflects the effect of unsettled security transactions and collateral on open derivative positions; at June 30, 2015, $159.7 million was included in "other liabilities," compared to $235.8 million and $31.3 million at June 30, 2014 and December 31, 2014

[2] The total fair value of the portfolio at June 30, 2015 and 2014, and December 31, 2014 included $0.7 billion, $1.1 billion, and $1.9 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of any unsettled security transactions.

[3] The $69.9 million decrease during the quarter reflects the reclassification of our 5% interest in ARX Holding Corp. upon acquisition of a controlling interest in ARX. The $39.4 million reflects our inception-to-date gain recognized, including the $2.0 million reduction in valuation that occurred during the second quarter 2015.

[4] 1The $69.3 million decrease during the year reflects the reclassification of our 5% interest in ARX Holding Corp. upon acquisition of a controlling interest in ARX. The $39.4 million reflects our inception-to-date gain recognized, including the $1.4 million reduction in valuation that occurred during the first six months of 2015.