Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	$ 41,079.3	$ 40,275.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6,455.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	91.2
Debt Securities, Available-for-sale, Total	47,443.7	43,011.7
U.S. Government and Agencies
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	1,246.1	1,702.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	156.0	123.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.4	10.9
Debt Securities, Available-for-sale, Total	1,401.7	1,814.4
State and Municipal Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	2,863.1	2,121.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	507.6	307.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	7.3	4.4
Debt Securities, Available-for-sale, Total	3,363.4	2,424.2
Foreign Governments
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	843.5	825.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	175.2	162.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1.0	4.8
Debt Securities, Available-for-sale, Total	1,017.7	983.7
Public Utilities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	6,436.7	6,626.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,303.7	850.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8.2	63.4
Debt Securities, Available-for-sale, Total	7,732.2	7,412.8
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	1,377.8	1,523.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	101.3	67.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.4	8.3
Debt Securities, Available-for-sale, Total	1,478.7	1,582.7
All Other Corporate Bonds
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	28,273.1	27,436.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4,211.2	1,981.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	73.9	664.4
Debt Securities, Available-for-sale, Total	32,410.4	28,754.0
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	39.0	39.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0.6	1.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0	0.2
Debt Securities, Available-for-sale, Total	39.6	39.9
Fixed Maturity Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	41,079.3	40,275.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6,455.6	3,492.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	91.2	756.4
Debt Securities, Available-for-sale, Total	$ 47,443.7	$ 43,011.7

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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