Amortized Cost and Fair Values of Securities by Security Type (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities
Total Amortized Cost of Fixed Maturity Securities	$ 39,025.9	$ 38,803.4
Fair Value of Fixed Maturity Securities	45,852.7	45,064.9
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Total Amortized Cost of Fixed Maturity Securities	1,140.5	983.5
Accumulated Gross Unrealized Gain on Securities	273.1	255.5
Accumulated Gross Unrealized Loss on Securities	0.2	0.5
Fair Value of Fixed Maturity Securities	1,413.4	1,238.5
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Total Amortized Cost of Fixed Maturity Securities	1,760.3	1,745.0
Accumulated Gross Unrealized Gain on Securities	402.0	377.6
Accumulated Gross Unrealized Loss on Securities	0.2	1.1
Fair Value of Fixed Maturity Securities	2,162.1	2,121.5
Foreign Governments
Schedule of Available-for-sale Securities
Total Amortized Cost of Fixed Maturity Securities	1,033.9	1,101.1
Accumulated Gross Unrealized Gain on Securities	210.0	206.3
Accumulated Gross Unrealized Loss on Securities	0	0
Fair Value of Fixed Maturity Securities	1,243.9	1,307.4
Public Utilities
Schedule of Available-for-sale Securities
Total Amortized Cost of Fixed Maturity Securities	7,044.5	7,046.1
Accumulated Gross Unrealized Gain on Securities	1,622.1	1,505.4
Accumulated Gross Unrealized Loss on Securities	0.5	0.9
Fair Value of Fixed Maturity Securities	8,666.1	8,550.6
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Total Amortized Cost of Fixed Maturity Securities	2,174.6	2,224.9
Accumulated Gross Unrealized Gain on Securities	240.8	207.0
Accumulated Gross Unrealized Loss on Securities	0.1	0.1
Fair Value of Fixed Maturity Securities	2,415.3	2,431.8
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Total Amortized Cost of Fixed Maturity Securities	25,828.1	25,658.8
Accumulated Gross Unrealized Gain on Securities	4,162.9	3,828.6
Accumulated Gross Unrealized Loss on Securities	88.9	122.2
Fair Value of Fixed Maturity Securities	29,902.1	29,365.2
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Total Amortized Cost of Fixed Maturity Securities	44.0	44.0
Accumulated Gross Unrealized Gain on Securities	5.8	5.9
Accumulated Gross Unrealized Loss on Securities	0	0
Fair Value of Fixed Maturity Securities	49.8	49.9
Fixed Maturity Securities
Schedule of Available-for-sale Securities
Total Amortized Cost of Fixed Maturity Securities	39,025.9	38,803.4
Accumulated Gross Unrealized Gain on Securities	6,916.7	6,386.3
Accumulated Gross Unrealized Loss on Securities	89.9	124.8
Fair Value of Fixed Maturity Securities	$ 45,852.7	$ 45,064.9

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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