Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Allocation of investments
Total fixed maturities	$ 4,603,534	$ 4,823,867
Short-term investments	725,258	273,514
Other investments	18,746	21,478
Recurring [Member]
Allocation of investments
Total fixed maturities	4,603,534	4,823,867
Short-term investments	725,258	273,514
Total	5,347,538	5,118,859
Recurring [Member] \| Level 1 [Member]
Allocation of investments
Total fixed maturities	0	0
Short-term investments	679,184	259,261
Total	679,184	259,261
Recurring [Member] \| Level 1 [Member] \| U.S. Government and Government Agency [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 1 [Member] \| Non-US Government and Government Agency [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 1 [Member] \| States, municipalities, political subdivision [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 1 [Member] \| Agency residential mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 1 [Member] \| Non-Agency residential mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 1 [Member] \| U.S. corporate [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 1 [Member] \| Non-U.S. corporate [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 1 [Member] \| Bank loans [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 1 [Member] \| Catastrophe bonds [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 1 [Member] \| Asset-backed securities [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 1 [Member] \| Commercial mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 1 [Member] \| Hedge funds [Member]
Allocation of investments
Other investments	0	0
Recurring [Member] \| Level 1 [Member] \| Mutual funds [Member]
Allocation of investments
Other investments	0	0
Recurring [Member] \| Level 2 [Member]
Allocation of investments
Total fixed maturities	4,603,534	4,823,867
Short-term investments	46,074	14,253
Total	4,658,578	4,846,706
Recurring [Member] \| Level 2 [Member] \| U.S. Government and Government Agency [Member]
Allocation of investments
Total fixed maturities	838,912	1,677,166
Recurring [Member] \| Level 2 [Member] \| Non-US Government and Government Agency [Member]
Allocation of investments
Total fixed maturities	476,590	554,199
Recurring [Member] \| Level 2 [Member] \| States, municipalities, political subdivision [Member]
Allocation of investments
Total fixed maturities	29,576	26,285
Recurring [Member] \| Level 2 [Member] \| Agency residential mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	470,933	445,859
Recurring [Member] \| Level 2 [Member] \| Non-Agency residential mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	51,223	56,470
Recurring [Member] \| Level 2 [Member] \| U.S. corporate [Member]
Allocation of investments
Total fixed maturities	1,406,591	1,308,406
Recurring [Member] \| Level 2 [Member] \| Non-U.S. corporate [Member]
Allocation of investments
Total fixed maturities	628,045	502,067
Recurring [Member] \| Level 2 [Member] \| Bank loans [Member]
Allocation of investments
Total fixed maturities	387,201	52,566
Recurring [Member] \| Level 2 [Member] \| Catastrophe bonds [Member]
Allocation of investments
Total fixed maturities	29,934	58,737
Recurring [Member] \| Level 2 [Member] \| Asset-backed securities [Member]
Allocation of investments
Total fixed maturities	276,273	123,569
Recurring [Member] \| Level 2 [Member] \| Commercial mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	8,256	18,543
Recurring [Member] \| Level 2 [Member] \| Hedge funds [Member]
Allocation of investments
Other investments	0	0
Recurring [Member] \| Level 2 [Member] \| Mutual funds [Member]
Allocation of investments
Other investments	8,970	8,586
Recurring [Member] \| Level 3 [Member]
Allocation of investments
Total fixed maturities	0	0
Short-term investments	0	0
Total	9,776	12,892
Recurring [Member] \| Level 3 [Member] \| U.S. Government and Government Agency [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 3 [Member] \| Non-US Government and Government Agency [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 3 [Member] \| States, municipalities, political subdivision [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 3 [Member] \| Agency residential mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 3 [Member] \| Non-Agency residential mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 3 [Member] \| U.S. corporate [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 3 [Member] \| Non-U.S. corporate [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 3 [Member] \| Bank loans [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 3 [Member] \| Catastrophe bonds [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 3 [Member] \| Asset-backed securities [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 3 [Member] \| Commercial mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	0	0
Recurring [Member] \| Level 3 [Member] \| Hedge funds [Member]
Allocation of investments
Other investments	9,776	12,892
Recurring [Member] \| Level 3 [Member] \| Mutual funds [Member]
Allocation of investments
Other investments	0	0
Recurring [Member] \| U.S. Government and Government Agency [Member]
Allocation of investments
Total fixed maturities	838,912	1,677,166
Recurring [Member] \| Non-US Government and Government Agency [Member]
Allocation of investments
Total fixed maturities	476,590	554,199
Recurring [Member] \| States, municipalities, political subdivision [Member]
Allocation of investments
Total fixed maturities	29,576	26,285
Recurring [Member] \| Agency residential mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	470,933	445,859
Recurring [Member] \| Non-Agency residential mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	51,223	56,470
Recurring [Member] \| U.S. corporate [Member]
Allocation of investments
Total fixed maturities	1,406,591	1,308,406
Recurring [Member] \| Non-U.S. corporate [Member]
Allocation of investments
Total fixed maturities	628,045	502,067
Recurring [Member] \| Bank loans [Member]
Allocation of investments
Total fixed maturities	387,201	52,566
Recurring [Member] \| Catastrophe bonds [Member]
Allocation of investments
Total fixed maturities	29,934	58,737
Recurring [Member] \| Asset-backed securities [Member]
Allocation of investments
Total fixed maturities	276,273	123,569
Recurring [Member] \| Commercial mortgage-backed securities [Member]
Allocation of investments
Total fixed maturities	8,256	18,543
Recurring [Member] \| Hedge funds [Member]
Allocation of investments
Other investments	9,776	12,892
Recurring [Member] \| Mutual funds [Member]
Allocation of investments
Other investments	$ 8,970	$ 8,586
Level 3 [Member]
Other Details
Percentage of total investments measured at fair value on recurring basis	0.20%	0.30%

Financial instruments 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, usually less than one year or the normal operating cycle, whichever is longer) or for debt and equity 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 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 12
-Subparagraph a
-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 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 159
-Paragraph 29
-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 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 13
-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 5: 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 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 7: 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.

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562

+ Details

- Definition

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.

+ References

Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 12
-Subparagraph a
-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 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 159
-Paragraph 29
-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 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.(a),1)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 13
-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
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562

Reference 8: 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.

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

+ Details

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Trading Securities which consist of all investments in certain debt and equity 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 which an entity has elected the fair value option. Trading generally reflects active and frequent buying and selling, and trading securities are generally used with the objective of generating profits on short-term differences in price, but this category of investments may also include items which are not intended to be sold in the near term or for which the fair value option has been elected.

+ References+ Details

- Definition

Total investments, other than debt and equity 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).

+ References+ Details