Amortized Cost and Fair Values of Securities by Security Type (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	$ 38,516.0	$ 38,289.6
Gross Unrealized Gain	5,290.6	4,358.7
Gross Unrealized Loss	139.8	303.9
Fixed Maturity Securities	43,666.8	42,344.4
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	959.1	1,028.6
Gross Unrealized Gain	200.1	173.1
Gross Unrealized Loss	2.5	5.6
Fixed Maturity Securities	1,156.7	1,196.1
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	1,732.3	1,706.0
Gross Unrealized Gain	212.2	117.2
Gross Unrealized Loss	17.7	40.0
Fixed Maturity Securities	1,926.8	1,783.2
Foreign Governments
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	1,205.7	1,226.4
Gross Unrealized Gain	166.3	149.6
Gross Unrealized Loss	1.0	2.8
Fixed Maturity Securities	1,371.0	1,373.2
Public Utilities
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	9,436.8	9,328.9
Gross Unrealized Gain	1,394.3	1,126.9
Gross Unrealized Loss	28.0	52.8
Fixed Maturity Securities	10,803.1	10,403.0
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	2,102.5	1,858.7
Gross Unrealized Gain	186.8	184.6
Gross Unrealized Loss	2.6	4.0
Fixed Maturity Securities	2,286.7	2,039.3
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	23,046.6	23,108.0
Gross Unrealized Gain	3,125.5	2,602.6
Gross Unrealized Loss	88.0	198.7
Fixed Maturity Securities	26,084.1	25,511.9
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	33.0	33.0
Gross Unrealized Gain	5.4	4.7
Gross Unrealized Loss	0	0
Fixed Maturity Securities	$ 38.4	$ 37.7