Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	$ 40,474.2	$ 40,275.2
Accumulated Gross Unrealized Gain on Securities	4,580.2
Accumulated Gross Unrealized Loss on Securities	271.8
Fixed Maturity Securities	44,782.6	43,011.7
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,714.2	1,702.1
Accumulated Gross Unrealized Gain on Securities	135.6	123.2
Accumulated Gross Unrealized Loss on Securities	4.0	10.9
Fixed Maturity Securities	1,845.8	1,814.4
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	2,151.0	2,121.5
Accumulated Gross Unrealized Gain on Securities	389.7	307.1
Accumulated Gross Unrealized Loss on Securities	0.3	4.4
Fixed Maturity Securities	2,540.4	2,424.2
Foreign Governments
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	822.1	825.8
Accumulated Gross Unrealized Gain on Securities	188.6	162.7
Accumulated Gross Unrealized Loss on Securities	3.0	4.8
Fixed Maturity Securities	1,007.7	983.7
Public Utilities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	6,451.6	6,626.2
Accumulated Gross Unrealized Gain on Securities	1,027.4	850.0
Accumulated Gross Unrealized Loss on Securities	24.5	63.4
Fixed Maturity Securities	7,454.5	7,412.8
Mortgage/Asset-Backed Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,473.6	1,523.8
Accumulated Gross Unrealized Gain on Securities	84.5	67.2
Accumulated Gross Unrealized Loss on Securities	2.0	8.3
Fixed Maturity Securities	1,556.1	1,582.7
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	27,822.7	27,436.8
Accumulated Gross Unrealized Gain on Securities	2,753.2	1,981.6
Accumulated Gross Unrealized Loss on Securities	238.0	664.4
Fixed Maturity Securities	30,337.9	28,754.0
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	39.0	39.0
Accumulated Gross Unrealized Gain on Securities	1.2	1.1
Accumulated Gross Unrealized Loss on Securities	0.0	0.2
Fixed Maturity Securities	40.2	39.9
Fixed Maturity Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	40,474.2	40,275.2
Accumulated Gross Unrealized Gain on Securities	4,580.2	3,492.9
Accumulated Gross Unrealized Loss on Securities	271.8	756.4
Fixed Maturity Securities	$ 44,782.6	$ 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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