Schedule VI - Supplementary Information Concerning Property-Casualty Insurance Operations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]
Deferred policy acquisition costs	$ 488,525	$ 452,101	$ 404,047
Reserves for losses and loss expenses	10,369,701	10,080,941	9,751,086
Unearned premium	3,026,732	2,781,437	2,474,847
Net premiums earned	5,744,418	5,226,537	4,673,516
Net investment income	600,885	544,291	586,763
Losses and loss expenses incurred:
Current year	3,495,825	3,221,393	2,997,995
Prior years		(75,764)	(78,810)	(102,571)
Loss reserve discount accretion	70,506	54,441	53,055
Amortization of deferred policy acquisition costs	1,053,397	991,070	917,583
Paid losses and loss expenses	3,115,227	2,878,071	2,708,935
Net premiums written	$ 5,996,947	$ 5,500,173	$ 4,898,539

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- Definition

Amount of amortization expense on deferred policy acquisition cost.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 235

-Section S99

-Paragraph 3

-Subparagraph (SX 210.12-18)

-URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e575016-122915

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 18

-Article 12

+ Details

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- Definition

Amount of insurance benefit, claims, loss and settlement expenses paid to the policyholders during the current accounting period.

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Amount of deferred policy acquisition cost related to property-casualty insurance policy written.

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- Definition

Amount of property-casualty insurance premium revenue earned on insurance policies.

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Amount of net investment income earned by property-casualty insurance underwriter.

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- Definition

Payments of claims and claims adjustment expenses during the period of a property-casualty insurance entity.

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Amount of actual premium written by property-casualty insurance underwriter.

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Amount of insurance benefit, claims, loss and settlement expenses paid to the policyholders during the prior accounting period.

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- Definition

The amount of reserve, policy claim payable and loss expense incurred by property-casualty insurance underwriter.

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Amount of property-casualty insurance premium revenue not yet earned on insurance policies.

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Supplemental Information for Property, Casualty Insurance Underwriters, Loss Reserve Discount Accretion

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