Investments Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	$ 40,804.8	$ 40,275.2
Accumulated Gross Unrealized Gain on Securities	5,783.8	3,492.9
Accumulated Gross Unrealized Loss on Securities	148.4	756.4
Fixed Maturity Securities	46,440.2	43,011.7
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	1,727.0	1,702.1
Accumulated Gross Unrealized Gain on Securities	159.2	123.2
Accumulated Gross Unrealized Loss on Securities	0.0	10.9
Fixed Maturity Securities	1,886.2	1,814.4
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	2,229.8	2,121.5
Accumulated Gross Unrealized Gain on Securities	478.5	307.1
Accumulated Gross Unrealized Loss on Securities	0.3	4.4
Fixed Maturity Securities	2,708.0	2,424.2
Foreign Governments
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	806.8	825.8
Accumulated Gross Unrealized Gain on Securities	199.5	162.7
Accumulated Gross Unrealized Loss on Securities	1.6	4.8
Fixed Maturity Securities	1,004.7	983.7
Public Utilities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	6,414.4	6,626.2
Accumulated Gross Unrealized Gain on Securities	1,217.5	850.0
Accumulated Gross Unrealized Loss on Securities	18.2	63.4
Fixed Maturity Securities	7,613.7	7,412.8
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	1,461.8	1,523.8
Accumulated Gross Unrealized Gain on Securities	105.1	67.2
Accumulated Gross Unrealized Loss on Securities	0.5	8.3
Fixed Maturity Securities	1,566.4	1,582.7
All Other Corporate Bonds
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	28,126.0	27,436.8
Accumulated Gross Unrealized Gain on Securities	3,623.0	1,981.6
Accumulated Gross Unrealized Loss on Securities	127.8	664.4
Fixed Maturity Securities	31,621.2	28,754.0
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	39.0	39.0
Accumulated Gross Unrealized Gain on Securities	1.0	1.1
Accumulated Gross Unrealized Loss on Securities	0.0	0.2
Fixed Maturity Securities	40.0	39.9
Fixed Maturity Securities
Debt Securities, Available-for-sale
Amortized Cost of Fixed Maturity Securities	40,804.8	40,275.2
Accumulated Gross Unrealized Gain on Securities	5,783.8	3,492.9
Accumulated Gross Unrealized Loss on Securities	148.4	756.4
Fixed Maturity Securities	$ 46,440.2	$ 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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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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