Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	$ 40,275.2	$ 39,780.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3,492.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	756.4
Fixed Maturity Securities	43,011.7	45,457.8
U.S. Government and Agencies
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	1,702.1	1,311.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	123.2	176.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	10.9	4.7
Fixed Maturity Securities	1,814.4	1,482.5
State and Municipal Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	2,121.5	1,942.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	307.1	395.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4.4	1.3
Fixed Maturity Securities	2,424.2	2,336.9
Foreign Governments
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	825.8	673.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	162.7	191.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4.8	0.4
Fixed Maturity Securities	983.7	863.9
Public Utilities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	6,626.2	6,952.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	850.0	1,296.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	63.4	12.6
Fixed Maturity Securities	7,412.8	8,236.5
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	1,523.8	1,873.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	67.2	105.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8.3	4.7
Fixed Maturity Securities	1,582.7	1,973.6
All Other Corporate Bonds
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	27,436.8	26,988.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,981.6	3,633.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	664.4	99.8
Fixed Maturity Securities	28,754.0	30,522.4
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	39.0	39.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1.1	3.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.2	0.0
Fixed Maturity Securities	39.9	42.0
Fixed Maturity Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	40,275.2	39,780.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3,492.9	5,800.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	756.4	123.5
Fixed Maturity Securities	$ 43,011.7	$ 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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