Amortized Cost and Fair Values of Securities by Security Type (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 37,751.5	[1]	$ 36,640.7
Gross Unrealized Gain	7,277.5		6,068.2
Gross Unrealized Loss	56.0		222.2
Fixed Maturity Securities	44,973.0		42,486.7
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,020.9	[1]	1,005.8
Gross Unrealized Gain	329.0		299.7
Gross Unrealized Loss	1.1		1.1
Fixed Maturity Securities	1,348.8		1,304.4
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,498.4	[1]	1,377.8
Gross Unrealized Gain	316.2		222.6
Gross Unrealized Loss	7.8		8.8
Fixed Maturity Securities	1,806.8		1,591.6
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,280.4	[1]	1,139.4
Gross Unrealized Gain	226.6		237.3
Gross Unrealized Loss	0		0
Fixed Maturity Securities	1,507.0		1,376.7
Public Utility, Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	9,294.3	[1]	9,015.7
Gross Unrealized Gain	1,865.0		1,646.2
Gross Unrealized Loss	15.1		28.6
Fixed Maturity Securities	11,144.2		10,633.3
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,927.9	[1]	2,634.6
Gross Unrealized Gain	289.1		344.1
Gross Unrealized Loss	0.5		5.5
Fixed Maturity Securities	2,216.5		2,973.2
All Other Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	22,696.6	[1]	21,411.6
Gross Unrealized Gain	4,245.3		3,314.8
Gross Unrealized Loss	31.5		176.3
Fixed Maturity Securities	26,910.4		24,550.1
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	33.0	[1]	55.8
Gross Unrealized Gain	6.3		3.5
Gross Unrealized Loss	0		1.9
Fixed Maturity Securities	$ 39.3		$ 57.4

[1] Amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from other-than-temporary declines in fair value, amortization of premiums, and accretion of discounts.