Commitments, Contingencies and Other Items (Narrative) (Details)	12 Months Ended
Dec. 31, 2018 USD ($)
Commitments and Contingencies [Line Items]
Initial commitments	$ 1,200,000,000
Fulfilled commitments	686,400,000
Unfunded commitments	$ 470,800,000
Initial capital lease term	20 years
Bargain renewal option term	30 years
Letter of Credit
Commitments and Contingencies [Line Items]
Letters of credit outstanding	$ 510,600,000
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
Investments as a Percentage of Shareholders' Equity
Commitments and Contingencies [Line Items]
Concentration risk percentage	10.00%

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Capital Lease, Term, Renewal Option

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Commitments and Contingencies [Line Items]

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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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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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The carrying amount, as of the date of the latest financial statement presented, of all assets that are pledged as collateral for the payment of the related debt obligation primarily arising from secured borrowings, repurchase agreements or other current or potential obligations, where the secured party does not have the right by contract or custom to sell or re-pledge the collateral and therefore are not reclassified and separately reported in the statement of financial position.

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