Related Party Transactions (Detail) (USD $)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Number Of Minority Owned Investment Management Company Affiliates	three				four
Premiums Written, Net	$ 1,383,475,000	$ 1,262,516,000	$ 4,585,682,000	$ 4,164,405,000
Reinsurance Contracts, ARX Holding Corporation
Premiums Written, Net	22,900,000	15,100,000
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	11,300,000	10,200,000
Related party reported acquisition costs	$ 6,700,000	$ 5,900,000

Premiums written for all insurance and reinsurance contracts, after adding premiums assumed from other insurers and subtracting any amounts ceded to another insurer.

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 Statement of Financial Accounting Standard (FAS)

-Number 113

-Paragraph 27

-Subparagraph a, b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 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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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration