Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Available-for-sale securities
Amortized cost, fixed maturities	$ 67,103.9	$ 68,797.3
Gross unrealized gains, fixed maturities	270.7	5,393.1
Gross unrealized losses, fixed maturities	7,743.2	288.6
Allowance, fixed maturities	0.1	4.9	$ 6.5
Fair value, fixed maturities	59,631.3	73,896.9
Accrued interest receivable	575.8	538.6
U.S. government and agencies
Available-for-sale securities
Amortized cost, fixed maturities	1,964.6	1,826.8
Gross unrealized gains, fixed maturities	0.1	144.5
Gross unrealized losses, fixed maturities	248.8	34.3
Fair value, fixed maturities	1,715.9	1,937.0
Non-U.S. governments
Available-for-sale securities
Amortized cost, fixed maturities	565.3	821.6
Gross unrealized gains, fixed maturities	18.9	127.5
Gross unrealized losses, fixed maturities	63.8	2.0
Fair value, fixed maturities	520.4	947.1
States and political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	7,280.1	8,210.3
Gross unrealized gains, fixed maturities	14.8	1,022.4
Gross unrealized losses, fixed maturities	1,126.6	16.3
Fair value, fixed maturities	6,168.3	9,216.4
Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	37,495.4	39,345.7
Gross unrealized gains, fixed maturities	219.5	3,750.2
Gross unrealized losses, fixed maturities	4,530.1	126.3
Allowance, fixed maturities		4.5
Fair value, fixed maturities	33,184.8	42,965.1
Residential mortgage-backed pass-through securities
Available-for-sale securities
Amortized cost, fixed maturities	2,362.1	2,296.9
Gross unrealized gains, fixed maturities	6.0	57.6
Gross unrealized losses, fixed maturities	197.2	12.2
Fair value, fixed maturities	2,170.9	2,342.3
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	5,529.7	5,388.6
Gross unrealized gains, fixed maturities	0.9	156.3
Gross unrealized losses, fixed maturities	703.1	30.9
Allowance, fixed maturities		0.3	4.3
Fair value, fixed maturities	4,827.5	5,513.7
Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	4,698.9	3,539.1
Gross unrealized gains, fixed maturities	4.5	4.0
Gross unrealized losses, fixed maturities	143.2	9.6
Allowance, fixed maturities			$ 2.2
Fair value, fixed maturities	4,560.2	3,533.5
Other debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	7,207.8	7,368.3
Gross unrealized gains, fixed maturities	6.0	130.6
Gross unrealized losses, fixed maturities	730.4	57.0
Allowance, fixed maturities	0.1	0.1
Fair value, fixed maturities	$ 6,483.3	$ 7,441.8

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).

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 https://asc.fasb.org/extlink&oid=126970911&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 https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

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

- Definition

Amount excluding accrued interest, 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 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

+ Details

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

- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

+ Details

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

- Definition

Amount excluding accrued interest, 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/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

+ Details

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

- Definition

Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(a)(4))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_InterestReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_MarketableSecuritiesLineItems
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_ForeignGovernmentDebtSecuritiesMember
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_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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