Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	$ 39,562.5	$ 39,658.7
Fair Value of Fixed Maturity Securities	45,862.0	43,354.4
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,265.5	1,265.8
Accumulated Gross Unrealized Gain on Securities	273.3	207.3
Accumulated Gross Unrealized Loss on Securities	0.0	7.9
Fair Value of Fixed Maturity Securities	1,538.8	1,465.2
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,806.3	1,828.3
Accumulated Gross Unrealized Gain on Securities	484.6	293.4
Accumulated Gross Unrealized Loss on Securities	0.1	4.6
Fair Value of Fixed Maturity Securities	2,290.8	2,117.1
Foreign Governments
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	805.2	897.2
Accumulated Gross Unrealized Gain on Securities	205.9	154.4
Accumulated Gross Unrealized Loss on Securities	0.0	0.0
Fair Value of Fixed Maturity Securities	1,011.1	1,051.6
Public Utilities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	6,931.5	6,979.3
Accumulated Gross Unrealized Gain on Securities	1,477.1	1,057.4
Accumulated Gross Unrealized Loss on Securities	2.1	16.3
Fair Value of Fixed Maturity Securities	8,406.5	8,020.4
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	2,261.1	2,318.6
Accumulated Gross Unrealized Gain on Securities	214.4	167.6
Accumulated Gross Unrealized Loss on Securities	0.3	4.7
Fair Value of Fixed Maturity Securities	2,475.2	2,481.5
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	26,448.9	26,325.5
Accumulated Gross Unrealized Gain on Securities	3,806.2	2,454.1
Accumulated Gross Unrealized Loss on Securities	162.6	608.2
Fair Value of Fixed Maturity Securities	30,092.5	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	0.5	0.6
Fair Value of Fixed Maturity Securities	47.1	47.2
Fixed Maturity Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	39,562.5	39,658.7
Accumulated Gross Unrealized Gain on Securities	6,465.1	4,338.0
Accumulated Gross Unrealized Loss on Securities	165.6	642.3
Fair Value of Fixed Maturity Securities	$ 45,862.0	$ 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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