Investments (Schedule of Fair Value of Fixed Maturity Investments Trading) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Investment [Line Items]
Fixed maturity investments trading, at fair value	$ 4,511,885	$ 4,665,421
U.S. treasuries [Member]
Investment [Line Items]
Fixed maturity investments trading, at fair value	1,186,742	1,259,800
Agencies [Member]
Investment [Line Items]
Fixed maturity investments trading, at fair value	285,969	315,154
Non-U.S. government (Sovereign Debt) [Member]
Investment [Line Items]
Fixed maturity investments trading, at fair value	199,561	133,198
Non-U.S. government-backed corporate [Member]
Investment [Line Items]
Fixed maturity investments trading, at fair value	291,077	349,514
Corporate [Member]
Investment [Line Items]
Fixed maturity investments trading, at fair value	1,598,749	1,607,233
Agency mortgage-backed [Member]
Investment [Line Items]
Fixed maturity investments trading, at fair value	433,661	399,619
Non-agency mortgage-backed [Member]
Investment [Line Items]
Fixed maturity investments trading, at fair value	223,392	230,747
Commercial mortgage-backed [Member]
Investment [Line Items]
Fixed maturity investments trading, at fair value	284,825	361,645
Asset-backed [Member]
Investment [Line Items]
Fixed maturity investments trading, at fair value	$ 7,909	$ 8,511

Debt securities that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for debt securities formerly categorized as available-for-sale or held-to-maturity which the entity held as of the date it opted to account for such securities at fair value. An enterprise may also categorize such a security as trading without the intent to sell it in the near term assuming the decision to categorize the security as trading occurred at acquisition.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

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-Section S99

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 159

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Implementation Guide (Q and A)

-Number FAS115

-Paragraph 35

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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