Summary of Shares Held as Treasury Stock and Related Carrying Value (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Equity, Class of Treasury Stock [Line Items]
Number of Treasury Shares	15,440	12,829	6,798
Treasury Shares Cost	$ 287,301	$ 225,034	$ 115,449

The amount allocated to treasury stock. Treasury stock is common and preferred shares of an entity that were issued, repurchased by the entity, and are held in its treasury.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 30

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6405834&loc=d3e23315-112656

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

-Publisher FASB

-Name FASB Technical Bulletin (FTB)

-Number 85-6

-Paragraph 3

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant