Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Cost	[1],[2]	$ 19,174.0	$ 17,402.8	$ 17,146.4
Gross Unrealized Gains	[1],[2]	1,506.7	1,621.4	1,623.4
Gross Unrealized Losses	[1],[2]	(104.8)	(49.2)	(30.6)
Net Realized Gains (Losses)	[1],[2],[3]	1.5	43.0	20.0
Total investments at fair value	[1],[2]	$ 20,577.4	$ 19,018.0	$ 18,759.2
% of Total Fair Value	[1],[2]	100.00%	100.00%	100.00%
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 15,520.6	$ 13,374.2	$ 12,265.4
Gross Unrealized Gains		152.0	207.1	257.0
Gross Unrealized Losses		(83.9)	(32.7)	(26.8)
Net Realized Gains (Losses)	[3]	0.3	0.6	3.0
Total investments at fair value		$ 15,589.0	$ 13,549.2	$ 12,498.6
% of Total Fair Value		75.70%	71.20%	66.60%
Fixed maturities | U.S. government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 2,049.3	$ 2,641.1	$ 3,249.0
Gross Unrealized Gains		14.0	27.3	43.5
Gross Unrealized Losses		(0.1)	(1.3)	(3.1)
Net Realized Gains (Losses)	[3]	0.0	0.0	0.0
Total investments at fair value		$ 2,063.2	$ 2,667.1	$ 3,289.4
% of Total Fair Value		10.00%	14.00%	17.50%
Fixed maturities | State and local government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 3,177.2	$ 2,095.7	$ 2,287.2
Gross Unrealized Gains		32.2	44.6	46.5
Gross Unrealized Losses		(17.1)	(1.1)	(2.6)
Net Realized Gains (Losses)	[3]	0.0	0.0	0.1
Total investments at fair value		$ 3,192.3	$ 2,139.2	$ 2,331.2
% of Total Fair Value		15.50%	11.20%	12.40%
Fixed maturities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 18.6	$ 14.2	$ 0.0
Gross Unrealized Gains		0.0	0.0	0.0
Gross Unrealized Losses		0.0	0.0	0.0
Net Realized Gains (Losses)	[3]	0.0	0.0	0.0
Total investments at fair value		$ 18.6	$ 14.2	$ 0.0
% of Total Fair Value		0.10%	0.10%	0.00%
Fixed maturities | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 3,433.9	$ 2,813.9	$ 2,215.4
Gross Unrealized Gains		23.3	32.9	47.9
Gross Unrealized Losses		(22.6)	(10.4)	(2.8)
Net Realized Gains (Losses)	[3]	0.2	0.3	2.3
Total investments at fair value		$ 3,434.8	$ 2,836.7	$ 2,262.8
% of Total Fair Value		16.70%	14.90%	12.10%
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 1,879.5	$ 1,635.5	$ 1,312.0
Gross Unrealized Gains		31.1	34.5	32.7
Gross Unrealized Losses		(17.6)	(10.8)	(9.3)
Net Realized Gains (Losses)	[3]	(0.5)	(0.7)	0.0
Total investments at fair value		$ 1,892.5	$ 1,658.5	$ 1,335.4
% of Total Fair Value		9.20%	8.70%	7.10%
Fixed maturities | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 116.0	$ 0.0	$ 0.0
Gross Unrealized Gains		0.1	0.0	0.0
Gross Unrealized Losses		(1.8)	0.0	0.0
Net Realized Gains (Losses)	[3]	0.0	0.0	0.0
Total investments at fair value		$ 114.3	$ 0.0	$ 0.0
% of Total Fair Value		0.60%	0.00%	0.00%
Fixed maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 2,548.4	$ 2,278.7	$ 1,974.4
Gross Unrealized Gains		26.0	39.3	47.8
Gross Unrealized Losses		(14.2)	(2.6)	(2.9)
Net Realized Gains (Losses)	[3]	(0.1)	0.2	0.0
Total investments at fair value		$ 2,560.1	$ 2,315.6	$ 2,019.3
% of Total Fair Value		12.40%	12.20%	10.80%
Fixed maturities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 2,037.9	$ 1,634.9	$ 965.4
Gross Unrealized Gains		3.6	3.8	7.1
Gross Unrealized Losses		(0.7)	(0.8)	(0.1)
Net Realized Gains (Losses)	[3]	0.7	0.8	0.6
Total investments at fair value		$ 2,041.5	$ 1,638.7	$ 973.0
% of Total Fair Value		9.90%	8.60%	5.20%
Fixed maturities | Redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 259.8	$ 260.2	$ 262.0
Gross Unrealized Gains		21.7	24.7	31.5
Gross Unrealized Losses		(9.8)	(5.7)	(6.0)
Net Realized Gains (Losses)	[3]	0.0	0.0	0.0
Total investments at fair value		$ 271.7	$ 279.2	$ 287.5
% of Total Fair Value		1.30%	1.50%	1.50%
Equity securities | Nonredeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 632.4	$ 590.4	$ 497.1
Gross Unrealized Gains		150.0	201.1	247.5
Gross Unrealized Losses		(11.3)	(6.4)	(1.4)
Net Realized Gains (Losses)	[3]	1.2	42.4	17.0
Total investments at fair value		$ 772.3	$ 827.5	$ 760.2
% of Total Fair Value		3.80%	4.40%	4.10%
Equity securities | Common equities
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 1,351.7	$ 1,289.2	$ 1,265.2
Gross Unrealized Gains		1,204.7	1,213.2	1,118.9
Gross Unrealized Losses		(9.6)	(10.1)	(2.4)
Net Realized Gains (Losses)	[3]	0.0	0.0	0.0
Total investments at fair value		$ 2,546.8	$ 2,492.3	$ 2,381.7
% of Total Fair Value		12.40%	13.10%	12.70%
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 1,669.3	$ 2,149.0	$ 3,118.7
Gross Unrealized Gains		0.0	0.0	0.0
Gross Unrealized Losses		0.0	0.0	0.0
Net Realized Gains (Losses)	[3]	0.0	0.0	0.0
Total investments at fair value		$ 1,669.3	$ 2,149.0	$ 3,118.7
% of Total Fair Value		8.10%	11.30%	16.60%

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