Reinsurance (Reinsurance Financial Information) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reinsurance Disclosures [Abstract]
Written premiums, Direct	$ 6,185,242us-gaap_DirectPremiumsWritten	$ 5,626,172us-gaap_DirectPremiumsWritten	$ 4,964,069us-gaap_DirectPremiumsWritten
Written premiums, Assumed	877,596us-gaap_AssumedPremiumsWritten	884,919us-gaap_AssumedPremiumsWritten	815,810us-gaap_AssumedPremiumsWritten
Written premiums, Ceded	(1,065,891)us-gaap_CededPremiumsWritten	(1,010,918)us-gaap_CededPremiumsWritten	(881,340)us-gaap_CededPremiumsWritten
Total net premiums written	5,996,947us-gaap_PremiumsWrittenNet	5,500,173us-gaap_PremiumsWrittenNet	4,898,539us-gaap_PremiumsWrittenNet
Earned premiums, Direct	5,889,021us-gaap_DirectPremiumsEarned	5,328,955us-gaap_DirectPremiumsEarned	4,723,882us-gaap_DirectPremiumsEarned
Earned premiums, Assumed	886,063us-gaap_AssumedPremiumsEarned	857,119us-gaap_AssumedPremiumsEarned	770,981us-gaap_AssumedPremiumsEarned
Earned premiums, Ceded	(1,030,666)us-gaap_CededPremiumsEarned	(959,537)us-gaap_CededPremiumsEarned	(821,347)us-gaap_CededPremiumsEarned
Net premiums earned	5,744,418us-gaap_PremiumsEarnedNet	5,226,537us-gaap_PremiumsEarnedNet	4,673,516us-gaap_PremiumsEarnedNet
Ceded losses incurred	475,802	556,108	528,018
Uncollectible reinsurance	1,144	1,385	1,680
Fees and Commissions, Other	$ 160,215us-gaap_FeesAndCommissionsOther	$ 137,449us-gaap_FeesAndCommissionsOther	$ 121,089us-gaap_FeesAndCommissionsOther

Amount of earned premiums assumed from other entities.

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

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Amount of written premiums assumed from other entities.

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Amount of earned premiums ceded to other entities.

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Amount of written premiums ceded to other entities.

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

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

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Fee and commission revenue earned from sources not otherwise specified in the existing taxonomy.

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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

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-Name Accounting Standards Codification

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Amount of reduction of provision for policy benefits and costs incurred for policies ceded.

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Amount of valuation allowance for reinsurance recoverables. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

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