Investment Portfolio by Major Security Type (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 16,655.4	[1],[2]	$ 15,138.2	[1],[2]	$ 15,789.7	[1],[2]
Gross Unrealized Gains	1,460.4	[1],[2]	1,361.3	[1],[2]	1,414.7	[1],[2]
Gross Unrealized Losses	(129.0)	[1],[2]	(34.0)	[1],[2]	(35.3)	[1],[2]
Net Realized Gains (Losses)	9.1	[1],[2],[3]	10.0	[1],[2],[3]	7.8	[1],[2],[3]
Total investments	17,995.9	[1],[2]	16,475.5	[1],[2]	17,176.9	[1],[2]
% of Total Fair Value	100.00%	[1],[2]	100.00%	[1],[2]	100.00%	[1],[2]
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost	13,644.9		11,373.9		11,864.0
Gross Unrealized Gains	271.1		417.7		454.1
Gross Unrealized Losses	(119.2)		(23.7)		(29.2)
Net Realized Gains (Losses)	0.9	[3]	6.2	[3]	6.3	[3]
Total investments	13,797.7		11,774.1		12,295.2
% of Total Fair Value	76.70%		71.50%		71.60%
Fixed maturities | U.S. government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	3,691.2		2,806.4		3,433.7
Gross Unrealized Gains	61.8		90.1		110.4
Gross Unrealized Losses	(11.3)		0		(0.1)
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments	3,741.7		2,896.5		3,544.0
% of Total Fair Value	20.80%		17.60%		20.60%
Fixed maturities | State and local government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	2,125.7		1,914.4		1,898.9
Gross Unrealized Gains	29.9		50.6		61.0
Gross Unrealized Losses	(17.0)		(0.6)		(0.4)
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments	2,138.6		1,964.4		1,959.5
% of Total Fair Value	11.90%		11.90%		11.40%
Fixed maturities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	15.9	[4]	0		0
Gross Unrealized Gains	0	[4]	0		0
Gross Unrealized Losses	0	[4]	0		0
Net Realized Gains (Losses)	0	[3],[4]	0	[3]	0	[3]
Total investments	15.9	[4]	0		0
% of Total Fair Value	0.10%	[4]	0.00%		0.00%
Fixed maturities | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	3,118.8		2,982.9		2,687.8
Gross Unrealized Gains	67.0		124.7		128.8
Gross Unrealized Losses	(28.9)		(1.0)		(0.4)
Net Realized Gains (Losses)	0.7	[3]	6.4	[3]	6.3	[3]
Total investments	3,157.6		3,113.0		2,822.5
% of Total Fair Value	17.60%		18.90%		16.40%
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	994.9		413.4		399.5
Gross Unrealized Gains	31.5		24.0		21.6
Gross Unrealized Losses	(16.5)		(9.2)		(12.4)
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments	1,009.9		428.2		408.7
% of Total Fair Value	5.60%		2.60%		2.40%
Fixed maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	2,224.1		1,963.9		1,998.2
Gross Unrealized Gains	49.0		84.9		90.7
Gross Unrealized Losses	(33.1)		(0.1)		(1.3)
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments	2,240.0		2,048.7		2,087.6
% of Total Fair Value	12.40%		12.40%		12.20%
Fixed maturities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	1,143.3		936.0		1,077.9
Gross Unrealized Gains	7.4		12.9		14.5
Gross Unrealized Losses	(2.0)		(0.1)		(0.1)
Net Realized Gains (Losses)	0.2	[3]	(0.2)	[3]	0	[3]
Total investments	1,148.9		948.6		1,092.3
% of Total Fair Value	6.40%		5.80%		6.40%
Fixed maturities | Redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	331.0		356.9		368.0
Gross Unrealized Gains	24.5		30.5		27.1
Gross Unrealized Losses	(10.4)		(12.7)		(14.5)
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments	345.1		374.7		380.6
% of Total Fair Value	1.90%		2.30%		2.20%
Equity securities | Nonredeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	442.1		404.0		430.1
Gross Unrealized Gains	280.5		404.6		416.8
Gross Unrealized Losses	(4.8)		0		0
Net Realized Gains (Losses)	8.2	[3]	3.8	[3]	1.5	[3]
Total investments	726.0		812.4		848.4
% of Total Fair Value	4.00%		4.90%		4.90%
Equity securities | Common equities
Schedule of Available-for-sale Securities [Line Items]
Cost	1,422.3		1,370.3		1,101.5
Gross Unrealized Gains	908.8		539.0		543.8
Gross Unrealized Losses	(5.0)		(10.3)		(6.1)
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments	2,326.1		1,899.0		1,639.2
% of Total Fair Value	12.90%		11.50%		9.60%
Short-term investments | Other short-term investments
Schedule of Available-for-sale Securities [Line Items]
Cost	1,146.1		1,990.0		2,394.1
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments	$ 1,146.1		$ 1,990.0		$ 2,394.1
% of Total Fair Value	6.40%		12.10%		13.90%

[1] The total fair value of the portfolio at September 30, 2013 and 2012, and December 31, 2012 included $1.3 billion, $1.6 billion, and $1.4 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company.

[2] At September 30, 2013, we had $63.7 million of net unsettled security transactions, including collateral on open derivative positions, recognized in other liabilities, compared to $32.4 million and $90.9 million recognized in other assets at September 30, 2012 and December 31, 2012, respectively.

[3] Represents net holding period gains (losses) on certain hybrid securities (discussed below).

[4] Reflects an Australian government obligation at September 30, 2013.