Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	$ 39,552.7	$ 39,658.7
Fair value of Fixed Maturity Securities	44,217.3	43,354.4
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,202.8	1,265.8
Accumulated Gross Unrealized Gain on Securities	183.1	207.3
Accumulated Gross Unrealized Loss on Securities	3.5	7.9
Fair value of Fixed Maturity Securities	1,382.4	1,465.2
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,868.0	1,828.3
Accumulated Gross Unrealized Gain on Securities	294.8	293.4
Accumulated Gross Unrealized Loss on Securities	4.8	4.6
Fair value of Fixed Maturity Securities	2,158.0	2,117.1
Foreign Governments
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	714.8	897.2
Accumulated Gross Unrealized Gain on Securities	199.9	154.4
Accumulated Gross Unrealized Loss on Securities	0.0	0.0
Fair value of Fixed Maturity Securities	914.7	1,051.6
Public Utilities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	6,916.1	6,979.3
Accumulated Gross Unrealized Gain on Securities	1,123.5	1,057.4
Accumulated Gross Unrealized Loss on Securities	16.9	16.3
Fair value of Fixed Maturity Securities	8,022.7	8,020.4
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	2,104.9	2,318.6
Accumulated Gross Unrealized Gain on Securities	134.7	167.6
Accumulated Gross Unrealized Loss on Securities	9.2	4.7
Fair value of Fixed Maturity Securities	2,230.4	2,481.5
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	26,707.1	26,325.5
Accumulated Gross Unrealized Gain on Securities	2,944.0	2,454.1
Accumulated Gross Unrealized Loss on Securities	183.9	608.2
Fair value of Fixed Maturity Securities	29,467.2	28,171.4
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	39.0	44.0
Accumulated Gross Unrealized Gain on Securities	3.2	3.8
Accumulated Gross Unrealized Loss on Securities	0.3	0.6
Fair value of Fixed Maturity Securities	41.9	47.2
Fixed Maturity Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	39,552.7	39,658.7
Accumulated Gross Unrealized Gain on Securities	4,883.2	4,338.0
Accumulated Gross Unrealized Loss on Securities	218.6	642.3
Fair value of Fixed Maturity Securities	$ 44,217.3	$ 43,354.4

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