Amortized Cost and Fair Values of Securities by Security Type (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 38,289.6	[1]	$ 37,751.5
Gross Unrealized Gain	4,358.7		7,277.5
Gross Unrealized Loss	303.9		56.0
Fixed Maturity Securities	42,344.4		44,973.0
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,028.6	[1]	1,020.9
Gross Unrealized Gain	173.1		329.0
Gross Unrealized Loss	5.6		1.1
Fixed Maturity Securities	1,196.1		1,348.8
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,706.0	[1]	1,498.4
Gross Unrealized Gain	117.2		316.2
Gross Unrealized Loss	40.0		7.8
Fixed Maturity Securities	1,783.2		1,806.8
Foreign Government Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,226.4	[1]	1,280.4
Gross Unrealized Gain	149.6		226.6
Gross Unrealized Loss	2.8		0
Fixed Maturity Securities	1,373.2		1,507.0
Public Utilities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	9,328.9	[1]	9,294.3
Gross Unrealized Gain	1,126.9		1,865.0
Gross Unrealized Loss	52.8		15.1
Fixed Maturity Securities	10,403.0		11,144.2
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,858.7	[1]	1,927.9
Gross Unrealized Gain	184.6		289.1
Gross Unrealized Loss	4.0		0.5
Fixed Maturity Securities	2,039.3		2,216.5
All Other Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	23,108.0	[1]	22,696.6
Gross Unrealized Gain	2,602.6		4,245.3
Gross Unrealized Loss	198.7		31.5
Fixed Maturity Securities	25,511.9		26,910.4
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	33.0	[1]	33.0
Gross Unrealized Gain	4.7		6.3
Gross Unrealized Loss	0		0
Fixed Maturity Securities	$ 37.7		$ 39.3

[1] (1)The 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/or accretion of discounts.