Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	$ 39,834.9	$ 39,552.7
Fair Value of Fixed Maturity Securities	45,422.6	44,217.3
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,316.2	1,202.8
Accumulated Gross Unrealized Gain on Securities	188.8	183.1
Accumulated Gross Unrealized Loss on Securities	3.8	3.5
Fair Value of Fixed Maturity Securities	1,501.2	1,382.4
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,913.7	1,868.0
Accumulated Gross Unrealized Gain on Securities	366.6	294.8
Accumulated Gross Unrealized Loss on Securities	1.7	4.8
Fair Value of Fixed Maturity Securities	2,278.6	2,158.0
Foreign Governments
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	768.9	714.8
Accumulated Gross Unrealized Gain on Securities	188.1	199.9
Accumulated Gross Unrealized Loss on Securities	1.1	0.0
Fair Value of Fixed Maturity Securities	955.9	914.7
Public Utilities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	6,949.4	6,916.1
Accumulated Gross Unrealized Gain on Securities	1,268.0	1,123.5
Accumulated Gross Unrealized Loss on Securities	9.5	16.9
Fair Value of Fixed Maturity Securities	8,207.9	8,022.7
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,925.7	2,104.9
Accumulated Gross Unrealized Gain on Securities	121.1	134.7
Accumulated Gross Unrealized Loss on Securities	3.6	9.2
Fair Value of Fixed Maturity Securities	2,043.2	2,230.4
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	26,922.0	26,707.1
Accumulated Gross Unrealized Gain on Securities	3,562.5	2,944.0
Accumulated Gross Unrealized Loss on Securities	90.9	183.9
Fair Value of Fixed Maturity Securities	30,393.6	29,467.2
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	39.0	39.0
Accumulated Gross Unrealized Gain on Securities	3.2	3.2
Accumulated Gross Unrealized Loss on Securities	0.0	0.3
Fair Value of Fixed Maturity Securities	42.2	41.9
Fixed Maturity Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	39,834.9	39,552.7
Accumulated Gross Unrealized Gain on Securities	5,698.3	4,883.2
Accumulated Gross Unrealized Loss on Securities	110.6	218.6
Fair Value of Fixed Maturity Securities	$ 45,422.6	$ 44,217.3

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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