Debt (Details) (USD $)	6 Months Ended			6 Months Ended					6 Months Ended														3 Months Ended	6 Months Ended
	Mar. 31, 2012 Trust	Sep. 30, 2011	Mar. 31, 2012 Unsecured intermediate debt issued August 15, 2002	Mar. 31, 2012 Unsecured intermediate debt issued August 15, 2002 Maximum	Mar. 31, 2012 Series C, due August 15, 2012, 6.46%	Sep. 30, 2011 Series C, due August 15, 2012, 6.46%	Mar. 31, 2012 Series D, due August 15, 2014, 6.56%	Sep. 30, 2011 Series D, due August 15, 2014, 6.56%	Mar. 31, 2012 Unsecured senior notes issued July 21, 2009 Installment	Mar. 31, 2012 Unsecured senior notes issued July 21, 2009 Maximum	Mar. 31, 2012 Unsecured senior notes issued July 21, 2009 Minimum Numerator Denominator	Mar. 31, 2012 Due July 21, 2012, 6.10%	Sep. 30, 2011 Due July 21, 2012, 6.10%	Mar. 31, 2012 Due July 21, 2013, 6.10%	Sep. 30, 2011 Due July 21, 2013, 6.10%	Mar. 31, 2012 Due July 21, 2014, 6.10%	Sep. 30, 2011 Due July 21, 2014, 6.10%	Mar. 31, 2012 Due July 21, 2015, 6.10%	Sep. 30, 2011 Due July 21, 2015, 6.10%	Mar. 31, 2012 Due July 21, 2016, 6.10%	Sep. 30, 2011 Due July 21, 2016, 6.10%	Mar. 31, 2012 Unsecured senior credit facility due December 18, 2011	Mar. 31, 2012 Letter of credit Location LetterOfCredit	Mar. 31, 2012 Letter of credit LetterOfCredit
Debt
Debt Instrument, Gross	$ 350,000,000	$ 350,000,000			$ 75,000,000	$ 75,000,000	$ 75,000,000	$ 75,000,000				$ 40,000,000	$ 40,000,000	$ 40,000,000	$ 40,000,000	$ 40,000,000	$ 40,000,000	$ 40,000,000	$ 40,000,000	$ 40,000,000	$ 40,000,000
Long-term debt due within one year	115,000,000	115,000,000
Long-term debt	235,000,000	235,000,000
Long-term debt stated interest rate percentage					6.46%		6.56%					6.10%		6.10%		6.10%		6.10%		6.10%
Weighted-average interest rate (as a percent)			6.53%
Ratio of debt to total capitalization (as a percent)				55.00%
Debt held by a company affiliated with one Board member			3,000,000
Face amount of the debt instrument									200,000,000
Number of equal annual principal repayments (in installments)									5
Annual principal repayments									40,000,000
Funded leverage ratio (as a percent)										55.00%
Interest coverage ratio, numerator											2.50
Interest coverage ratio, denominator											1.00
Number of stand-by letters of credit																							5
Letters of credit issued by a bank to support customs and transportation guarantees																							3,000,000	3,000,000
Number of international locations between which a rig is moved																							2
Maximum borrowing capacity allowed to expire at maturity																						400,000,000
Number of funded collateral trusts	2
Number of letters of credit terminated																								2
Funded collateral trusts	$ 26,100,000