Investments Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 39,913.2	$ 39,780.5
Accumulated Gross Unrealized Gain on Securities	3,654.7
Accumulated Gross Unrealized Loss on Securities	496.6
Fixed Maturity Securities	43,071.3	45,457.8
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	1,691.7	1,311.1
Accumulated Gross Unrealized Gain on Securities	113.4	176.1
Accumulated Gross Unrealized Loss on Securities	25.6	4.7
Fixed Maturity Securities	1,779.5	1,482.5
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	2,049.2	1,942.8
Accumulated Gross Unrealized Gain on Securities	280.5	395.4
Accumulated Gross Unrealized Loss on Securities	9.7	1.3
Fixed Maturity Securities	2,320.0	2,336.9
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	644.1	673.0
Accumulated Gross Unrealized Gain on Securities	156.0	191.3
Accumulated Gross Unrealized Loss on Securities	1.7	0.4
Fixed Maturity Securities	798.4	863.9
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	6,592.2	6,952.7
Accumulated Gross Unrealized Gain on Securities	851.1	1,296.4
Accumulated Gross Unrealized Loss on Securities	53.4	12.6
Fixed Maturity Securities	7,389.9	8,236.5
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	1,591.0	1,873.2
Accumulated Gross Unrealized Gain on Securities	57.1	105.1
Accumulated Gross Unrealized Loss on Securities	21.1	4.7
Fixed Maturity Securities	1,627.0	1,973.6
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	27,306.0	26,988.7
Accumulated Gross Unrealized Gain on Securities	2,195.0	3,633.5
Accumulated Gross Unrealized Loss on Securities	384.9	99.8
Fixed Maturity Securities	29,116.1	30,522.4
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	39.0	39.0
Accumulated Gross Unrealized Gain on Securities	1.6	3.0
Accumulated Gross Unrealized Loss on Securities	0.2	0.0
Fixed Maturity Securities	40.4	42.0
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	39,913.2	39,780.5
Accumulated Gross Unrealized Gain on Securities	3,654.7	5,800.8
Accumulated Gross Unrealized Loss on Securities	496.6	123.5
Fixed Maturity Securities	$ 43,071.3	$ 45,457.8

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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