Commitments, Contingencies and Other Items (Narrative) (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Commitments and Contingencies [Line Items]
Initial commitments	$ 1,800,000,000
Fulfilled commitments	809,000,000.0
Unfunded commitments	$ 1,000,000,000.0
Operating lease, weighted average lease term	3 years 8 months 12 days
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssets	us-gaap:OtherAssets
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilities	us-gaap:OtherLiabilities
Operating lease, right-of-use asset	$ 29,800,000	$ 42,800,000
Operating lease, liability	29,900,000	42,900,000
Operating lease, expense	$ 9,800,000	$ 8,500,000
Lessee, finance lease, term of contract	20 years
Lessee, finance lease, renewal term	30 years
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssets	us-gaap:OtherAssets
Finance Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilities	us-gaap:OtherLiabilities
Letter of Credit
Commitments and Contingencies [Line Items]
Letters of credit outstanding	$ 1,100,000,000
Letter of Credit \| Top Layer Reinsurance Ltd.
Commitments and Contingencies [Line Items]
Collateral pledged to support letter of credit	37,500,000
Letter of Credit \| Capital Support Agreement \| Top Layer Reinsurance Ltd.
Commitments and Contingencies [Line Items]
Maximum mandatory capital contribution in the event of loss that reduced capital and surplus below specified level	$ 50,000,000.0
Investments as a Percentage of Shareholders' Equity
Commitments and Contingencies [Line Items]
Concentration risk percentage	10.00%
Other assets
Commitments and Contingencies [Line Items]
Finance lease, right-of-use asset	$ 18,000,000.0	$ 19,800,000
Other liabilities
Commitments and Contingencies [Line Items]
Finance lease, liability	22,900,000	25,100,000
Other income (loss)
Commitments and Contingencies [Line Items]
Finance lease, right-of-use asset, amortization	500,000	900,000
Interest Expense
Commitments and Contingencies [Line Items]
Finance lease, interest expense	$ 2,300,000	$ 2,600,000

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Private Equity Investments and Other Entities, Commitments Fulfilled

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Private Equity Investments and Other Entities, Initial Commitments

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Private Equity Investments and Other Entities, Unfunded Commitments

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount of interest expense on finance lease liability.

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Present value of lessee's discounted obligation for lease payments from finance lease.

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Indicates line item in statement of financial position that includes finance lease liability.

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Amount, after accumulated amortization, of right-of-use asset from finance lease.

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Amount of amortization expense attributable to right-of-use asset from finance lease.

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Indicates line item in statement of financial position that includes finance lease right-of-use asset.

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Term of lessee's finance lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Term of lessee's finance lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

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Amount of operating lease expense. Excludes sublease income.

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Present value of lessee's discounted obligation for lease payments from operating lease.

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Indicates line item in statement of financial position that includes operating lease liability.

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Amount of lessee's right to use underlying asset under operating lease.

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Indicates line item in statement of financial position that includes operating lease right-of-use asset.

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Weighted average remaining lease term for operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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