Schedule II. Condensed Financial Information Of Registrant (Balance Sheets Parenthetical) (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Fixed maturity investments trading, amortized cost	$ 7,434,870	$ 6,920,690
Preference shares, par value (in usd per share)	$ 1	$ 1
Preference shares, shares issued (in shares)	16,000,000	16,000,000
Preference shares, shares outstanding (in shares)	16,000,000	16,000,000
Common shares, par value (in usd per share)	$ 1	$ 1
Common shares, shares issued (in shares)	40,023,789	41,187,413
Common shares, shares outstanding (in shares)	40,023,789	41,187,413
Parent Company
Condensed Financial Statements, Captions [Line Items]
Fixed maturity investments trading, amortized cost	$ 104,521	$ 22,402
Preference shares, par value (in usd per share)	$ 1.00	$ 1.00
Preference shares, shares issued (in shares)	16,000,000	16,000,000
Preference shares, shares outstanding (in shares)	16,000,000	16,000,000
Common shares, par value (in usd per share)	$ 1.00	$ 1.00
Common shares, shares issued (in shares)	40,023,789	41,187,413
Common shares, shares outstanding (in shares)	40,023,789	41,187,413

Face amount or stated value per share of common stock.

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644

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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.

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No definition available.

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Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

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Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

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Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.

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Amortized cost of investments in debt securities classified as trading.

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