Amortized Cost and Fair Values of Securities by Security Type (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	$ 38,920.4	$ 38,289.6
Gross Unrealized Gain	5,879.4	4,358.7
Gross Unrealized Loss	84.6	303.9
Fixed Maturity Securities	44,715.2	42,344.4
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	975.3	1,028.6
Gross Unrealized Gain	222.5	173.1
Gross Unrealized Loss	1.4	5.6
Fixed Maturity Securities	1,196.4	1,196.1
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,744.0	1,706.0
Gross Unrealized Gain	302.4	117.2
Gross Unrealized Loss	3.8	40.0
Fixed Maturity Securities	2,042.6	1,783.2
Foreign Governments
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,166.9	1,226.4
Gross Unrealized Gain	179.8	149.6
Gross Unrealized Loss	0	2.8
Fixed Maturity Securities	1,346.7	1,373.2
Public Utilities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	7,121.4	7,121.7
Gross Unrealized Gain	1,317.8	901.2
Gross Unrealized Loss	4.6	25.7
Fixed Maturity Securities	8,434.6	7,997.2
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	2,309.8	1,858.7
Gross Unrealized Gain	192.5	184.6
Gross Unrealized Loss	0.6	4.0
Fixed Maturity Securities	2,501.7	2,039.3
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	25,559.0	25,315.2
Gross Unrealized Gain	3,658.3	2,828.3
Gross Unrealized Loss	74.2	225.8
Fixed Maturity Securities	29,143.1	27,917.7
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	44.0	33.0
Gross Unrealized Gain	6.1	4.7
Gross Unrealized Loss	0	0
Fixed Maturity Securities	$ 50.1	$ 37.7