Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	$ 39,804.7	$ 39,658.7
Fair Value of Fixed Maturity Securities	44,699.2	43,354.4
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,338.7	1,265.8
Accumulated Gross Unrealized Gain on Securities	247.7	207.3
Accumulated Gross Unrealized Loss on Securities	0.1	7.9
Fair Value of Fixed Maturity Securities	1,586.3	1,465.2
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,814.0	1,828.3
Accumulated Gross Unrealized Gain on Securities	364.0	293.4
Accumulated Gross Unrealized Loss on Securities	1.4	4.6
Fair Value of Fixed Maturity Securities	2,176.6	2,117.1
Foreign Governments
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	895.7	897.2
Accumulated Gross Unrealized Gain on Securities	182.2	154.4
Accumulated Gross Unrealized Loss on Securities	0.0	0.0
Fair Value of Fixed Maturity Securities	1,077.9	1,051.6
Public Utilities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	6,974.0	6,979.3
Accumulated Gross Unrealized Gain on Securities	1,273.0	1,057.4
Accumulated Gross Unrealized Loss on Securities	3.8	16.3
Fair Value of Fixed Maturity Securities	8,243.2	8,020.4
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	2,279.8	2,318.6
Accumulated Gross Unrealized Gain on Securities	201.0	167.6
Accumulated Gross Unrealized Loss on Securities	0.4	4.7
Fair Value of Fixed Maturity Securities	2,480.4	2,481.5
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	26,458.5	26,325.5
Accumulated Gross Unrealized Gain on Securities	3,047.5	2,454.1
Accumulated Gross Unrealized Loss on Securities	418.5	608.2
Fair Value of Fixed Maturity Securities	29,087.5	28,171.4
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	44.0	44.0
Accumulated Gross Unrealized Gain on Securities	3.6	3.8
Accumulated Gross Unrealized Loss on Securities	0.3	0.6
Fair Value of Fixed Maturity Securities	47.3	47.2
Fixed Maturity Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	39,804.7	39,658.7
Accumulated Gross Unrealized Gain on Securities	5,319.0	4,338.0
Accumulated Gross Unrealized Loss on Securities	424.5	642.3
Fair Value of Fixed Maturity Securities	$ 44,699.2	$ 43,354.4

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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