Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	$ 40,840.4		$ 41,079.3
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	29.8		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7,654.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	231.4
Debt Securities, Available-for-sale, Total	48,233.4		47,443.7
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	1,140.0		1,246.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	226.3		156.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0		0.4
Debt Securities, Available-for-sale, Total	1,366.3		1,401.7
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	2,901.4		2,863.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	585.6		507.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4.1		7.3
Debt Securities, Available-for-sale, Total	3,482.9		3,363.4
Foreign Governments
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	820.7		843.5
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	249.5		175.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1.4		1.0
Debt Securities, Available-for-sale, Total	1,068.8		1,017.7
Public Utilities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	6,331.4		6,436.7
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,598.8		1,303.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	7.2		8.2
Debt Securities, Available-for-sale, Total	7,923.0		7,732.2
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	1,334.7		1,377.8
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	133.3		101.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.2		0.4
Debt Securities, Available-for-sale, Total	1,467.8		1,478.7
All Other Corporate Bonds
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	28,302.6		28,273.1
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	29.8	$ 48.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4,860.7		4,211.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	218.3		73.9
Debt Securities, Available-for-sale, Total	32,915.2		32,410.4
Redeemable Preferred Stocks
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	9.6		39.0
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0.0		0.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.2		0.0
Debt Securities, Available-for-sale, Total	9.4		39.6
Fixed Maturity Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost, Total	40,840.4		41,079.3
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	29.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7,654.2		6,455.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	231.4		91.2
Debt Securities, Available-for-sale, Total	$ 48,233.4		$ 47,443.7

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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_PublicUtilityBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AllOtherCorporateBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_RedeemablePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: