Investments Investment Portfolio by Major Security Type (Detail) (USD $)	Jun. 30, 2014		Dec. 31, 2013		Jun. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 17,146,400,000	[1],[2]	$ 16,584,700,000	[1],[2]	$ 16,237,200,000	[1],[2]
Gross Unrealized Gains	1,623,400,000	[1],[2]	1,582,900,000	[1],[2]	1,389,100,000	[1],[2]
Gross Unrealized Losses	(30,600,000)	[1],[2]	(126,000,000)	[1],[2]	(146,700,000)	[1],[2]
Net Realized Gains (Losses)	20,000,000	[1],[2],[3]	13,100,000	[1],[2],[3]	6,800,000	[1],[2],[3]
Total investments at fair value	18,759,200,000	[1],[2]	18,054,700,000	[1],[2]	17,486,400,000	[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	12,265,400,000		13,415,300,000		12,824,100,000
Gross Unrealized Gains	257,000,000		242,400,000		262,500,000
Gross Unrealized Losses	(26,800,000)		(119,100,000)		(137,000,000)
Net Realized Gains (Losses)	3,000,000	[3]	1,800,000	[3]	(200,000)	[3]
Total investments at fair value	12,498,600,000		13,540,400,000		12,949,400,000
% of Total Fair Value	66.60%		75.00%		74.10%
Fixed maturities | U.S. government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	3,249,000,000		3,630,400,000		3,362,800,000
Gross Unrealized Gains	43,500,000		48,400,000		63,000,000
Gross Unrealized Losses	(3,100,000)		(16,600,000)		(15,000,000)
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments at fair value	3,289,400,000		3,662,200,000		3,410,800,000
% of Total Fair Value	17.50%		20.30%		19.50%
Fixed maturities | State and local government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	2,287,200,000		2,247,300,000		2,010,600,000
Gross Unrealized Gains	46,500,000		27,100,000		27,700,000
Gross Unrealized Losses	(2,600,000)		(18,400,000)		(18,400,000)
Net Realized Gains (Losses)	100,000	[3]	0	[3]	0	[3]
Total investments at fair value	2,331,200,000		2,256,000,000		2,019,900,000
% of Total Fair Value	12.40%		12.50%		11.60%
Fixed maturities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	0		15,600,000		16,400,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments at fair value	0		15,600,000		16,400,000
% of Total Fair Value	0.00%		0.10%		0.10%
Fixed maturities | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	2,215,400,000		2,885,000,000		3,048,800,000
Gross Unrealized Gains	47,900,000		60,400,000		57,000,000
Gross Unrealized Losses	(2,800,000)		(20,400,000)		(33,000,000)
Net Realized Gains (Losses)	2,300,000	[3]	1,600,000	[3]	(800,000)	[3]
Total investments at fair value	2,262,800,000		2,926,600,000		3,072,000,000
% of Total Fair Value	12.10%		16.20%		17.60%
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	1,312,000,000		1,110,100,000		870,900,000
Gross Unrealized Gains	32,700,000		31,900,000		30,500,000
Gross Unrealized Losses	(9,300,000)		(14,100,000)		(14,700,000)
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments at fair value	1,335,400,000		1,127,900,000		886,700,000
% of Total Fair Value	7.10%		6.20%		5.10%
Fixed maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	1,974,400,000		2,154,400,000		2,213,900,000
Gross Unrealized Gains	47,800,000		43,900,000		48,900,000
Gross Unrealized Losses	(2,900,000)		(37,800,000)		(41,000,000)
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments at fair value	2,019,300,000		2,160,500,000		2,221,800,000
% of Total Fair Value	10.80%		12.00%		12.70%
Fixed maturities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	965,400,000		1,073,000,000		941,700,000
Gross Unrealized Gains	7,100,000		6,600,000		7,600,000
Gross Unrealized Losses	(100,000)		(2,100,000)		(2,200,000)
Net Realized Gains (Losses)	600,000	[3]	200,000	[3]	600,000	[3]
Total investments at fair value	973,000,000		1,077,700,000		947,700,000
% of Total Fair Value	5.20%		6.00%		5.40%
Fixed maturities | Redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	262,000,000		299,500,000		359,000,000
Gross Unrealized Gains	31,500,000		24,100,000		27,800,000
Gross Unrealized Losses	(6,000,000)		(9,700,000)		(12,700,000)
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments at fair value	287,500,000		313,900,000		374,100,000
% of Total Fair Value	1.50%		1.70%		2.10%
Equity securities | Nonredeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	497,100,000		445,700,000		416,800,000
Gross Unrealized Gains	247,500,000		258,700,000		332,100,000
Gross Unrealized Losses	(1,400,000)		(4,500,000)		(1,500,000)
Net Realized Gains (Losses)	17,000,000	[3]	11,300,000	[3]	7,000,000	[3]
Total investments at fair value	760,200,000		711,200,000		754,400,000
% of Total Fair Value	4.10%		3.90%		4.30%
Equity securities | Common equities
Schedule of Available-for-sale Securities [Line Items]
Cost	1,265,200,000		1,451,100,000		1,438,200,000
Gross Unrealized Gains	1,118,900,000		1,081,800,000		794,500,000
Gross Unrealized Losses	(2,400,000)		(2,400,000)		(8,200,000)
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments at fair value	2,381,700,000		2,530,500,000		2,224,500,000
% of Total Fair Value	12.70%		14.00%		12.70%
Short-term investments | Other short-term investments
Schedule of Available-for-sale Securities [Line Items]
Cost	3,118,700,000		1,272,600,000		1,558,100,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Net Realized Gains (Losses)	0	[3]	0	[3]	0	[3]
Total investments at fair value	$ 3,118,700,000		$ 1,272,600,000		$ 1,558,100,000
% of Total Fair Value	16.60%		7.10%		8.90%

[1] Our portfolio reflects the effect of unsettled security transactions and collateral on open derivative positions; at June 30, 2014, $235.8 million was included in "other liabilities," compared to $117.0 million and $61.3 million at June 30, 2013 and December 31, 2013, respectively.

[2] The total fair value of the portfolio at June 30, 2014 and 2013, and December 31, 2013 included $1.1 billion, $1.2 billion, and $1.8 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).