Debt (Details) (USD $)	3 Months Ended				9 Months Ended					9 Months Ended													1 Months Ended	9 Months Ended		1 Months Ended			1 Months Ended		3 Months Ended		9 Months Ended
	Dec. 31, 2011 item	Jun. 30, 2012	Sep. 30, 2011	Jun. 30, 2012 Unsecured intermediate debt issued August 15, 2002	Jun. 30, 2012 Unsecured intermediate debt issued August 15, 2002 Maximum	Jun. 30, 2012 Series C, due August 15, 2012, 6.46%	Sep. 30, 2011 Series C, due August 15, 2012, 6.46%	Jun. 30, 2012 Series D, due August 15, 2014, 6.56%	Sep. 30, 2011 Series D, due August 15, 2014, 6.56%	Jun. 30, 2012 Unsecured senior notes issued July 21, 2009 item	Jun. 30, 2012 Unsecured senior notes issued July 21, 2009 Maximum	Jun. 30, 2012 Unsecured senior notes issued July 21, 2009 Minimum	Jun. 30, 2012 Due July 21, 2012, 6.10%	Sep. 30, 2011 Due July 21, 2012, 6.10%	Jun. 30, 2012 Due July 21, 2013, 6.10%	Sep. 30, 2011 Due July 21, 2013, 6.10%	Jun. 30, 2012 Due July 21, 2014, 6.10%	Sep. 30, 2011 Due July 21, 2014, 6.10%	Jun. 30, 2012 Due July 21, 2015, 6.10%	Sep. 30, 2011 Due July 21, 2015, 6.10%	Jun. 30, 2012 Due July 21, 2016, 6.10%	Sep. 30, 2011 Due July 21, 2016, 6.10%	May 31, 2012 Unsecured senior credit facility mature May 2017	Jun. 30, 2012 Unsecured senior credit facility mature May 2017	May 25, 2012 Unsecured senior credit facility mature May 2017	May 31, 2012 Unsecured senior credit facility mature May 2017 Maximum	Jun. 30, 2012 Unsecured senior credit facility mature May 2017 Maximum	May 25, 2012 Unsecured senior credit facility mature May 2017 Maximum	May 31, 2012 Unsecured senior credit facility mature May 2017 Minimum	May 25, 2012 Unsecured senior credit facility mature May 2017 Minimum	Mar. 31, 2012 Letter of credit item	Dec. 31, 2011 Letter of credit LetterOfCredit	Jun. 30, 2012 Letter of credit item	Dec. 31, 2011 Unsecured senior credit facility due December 18, 2011
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%		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%																50.00%
Interest coverage ratio												2.50															3.00
Number of stand-by letters of credit																															5		2
Letters of credit issued by a bank to support customs and transportation guarantees																															3,000,000		200,000
Number of international locations between which a rig is moved																															2
Letters of credit outstanding																																	600,000
Outstanding amount under letters of credit issued by bank to support customs and transportation guarantees																																	200,000
Borrowing amount																									300,000,000
Base rate for the variable rate																							LIBOR
LIBOR spread on borrowings (as a percent)																								1.125%				1.75%		1.125%
Commitment fee (as a percent)																								0.15%		0.35%			0.15%
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