Schedule IV - Reinsurance (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	$ 6,185,242us-gaap_DirectPremiumsWritten	$ 5,626,172us-gaap_DirectPremiumsWritten	$ 4,964,069us-gaap_DirectPremiumsWritten
Ceded to Other Companies	1,065,891us-gaap_CededPremiumsWritten	1,010,918us-gaap_CededPremiumsWritten	881,340us-gaap_CededPremiumsWritten
Assumed from Other Companies	877,596us-gaap_AssumedPremiumsWritten	884,919us-gaap_AssumedPremiumsWritten	815,810us-gaap_AssumedPremiumsWritten
Total net premiums written	5,996,947us-gaap_PremiumsWrittenNet	5,500,173us-gaap_PremiumsWrittenNet	4,898,539us-gaap_PremiumsWrittenNet
Percentage of Amount Assumed to Net	14.60%us-gaap_PremiumsPercentageAssumedToNet	16.10%us-gaap_PremiumsPercentageAssumedToNet	16.70%us-gaap_PremiumsPercentageAssumedToNet
Insurance-Domestic
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	5,214,849	4,699,348	4,133,960
Ceded to Other Companies	866,092	809,368	691,284
Assumed from Other Companies	168,830	104,407	127,207
Total net premiums written	4,517,587	3,994,387	3,569,883
Percentage of Amount Assumed to Net	3.70%	2.60%	3.60%
Insurance-International
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	927,799	887,027	798,750
Ceded to Other Companies	156,195	142,591	137,598
Assumed from Other Companies	56,472	11,749	3,307
Total net premiums written	828,076	756,185	664,459
Percentage of Amount Assumed to Net	6.80%	1.60%	0.50%
Reinsurance-Global
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Amount	42,594	39,797	31,359
Ceded to Other Companies	43,604	58,959	52,458
Assumed from Other Companies	652,294	768,763	685,296
Total net premiums written	$ 651,284	$ 749,601	$ 664,197
Percentage of Amount Assumed to Net	100.20%	102.60%	103.20%

Amount of written premiums assumed from other entities.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-Subparagraph (b)(2),(c)(2)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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Period Type:	duration

- Definition

Amount of written premiums ceded to other entities.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-Subparagraph (b),(c)(2)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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- Definition

Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-Subparagraph (b),(c)(1)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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Period Type:	duration

- Definition

Percentage of assumed premiums earned by the entity to net premiums earned.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 235

-Section S99

-Paragraph 2

-Subparagraph (SX 210.12-17)

-URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e574992-122915

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- Definition

Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-Subparagraph (b),(c)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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