Fair Value Measurements - Changes in Level 3 Securities (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended			6 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Auction Rate Securities [Member]	Jun. 30, 2012 Auction Rate Securities [Member]	Dec. 31, 2011 Auction Rate Securities [Member]	Jun. 30, 2013 Obligations Issued by States and Political Subdivisions [Member]	Jun. 30, 2012 Obligations Issued by States and Political Subdivisions [Member]	Jun. 30, 2013 Equity Securities [Member]	Jun. 30, 2012 Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 53,782	$ 18,914	$ 3,963	$ 3,725	$ 3,725	$ 49,477	$ 14,772	$ 342	$ 417
Purchases	29,370	64,970				29,370	64,970
Maturities and calls	(30,840)	(15,092)				(30,830)	(15,058)	(10)	(34)
Amortization	(18)	(15)				(18)	(15)
Changes in fair value
Ending Balance	$ 52,294	$ 68,777	$ 3,963	$ 3,725	$ 3,725	$ 47,999	$ 64,669	$ 332	$ 383

Fair value measurement with unobservable inputs reconciliation recurring basis asset amortization.

No definition available.

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- Definition

Fair value measurement with unobservable inputs reconciliation recurring basis asset change in fair value.

+ References

No definition available.

+ Details

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- Definition

Fair value measurement with unobservable inputs reconciliation recurring basis asset maturities.

+ References

No definition available.

+ Details

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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