Principal Amounts Carrying Amounts and Fair Values of Senior Notes (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012 May 2008 senior notes	Sep. 30, 2011 May 2008 senior notes	May 07, 2008 May 2008 senior notes	Sep. 30, 2012 July 2010 senior notes	Sep. 30, 2011 July 2010 senior notes	Jul. 14, 2010 July 2010 senior notes
Debt Instrument [Line Items]
Principal	$ 259,000	$ 267,000	$ 275,000	$ 245,000	$ 245,000	$ 245,000
Carrying Amounts	259,000	267,000		245,000	245,000
Fair Value	$ 275,201	$ 305,874		$ 243,866	$ 266,620

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16, 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant