Debt (Details) (USD $)	3 Months Ended					3 Months Ended												3 Months Ended
	Dec. 31, 2011	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2012 Series D, due August 15, 2014, 6.56%	Sep. 30, 2012 Series D, due August 15, 2014, 6.56%	Dec. 31, 2012 Series D, due August 15, 2014, 6.56% Maximum	Dec. 31, 2012 Unsecured senior notes issued July 21, 2009	Dec. 31, 2012 Unsecured senior notes issued July 21, 2009 Maximum	Dec. 31, 2012 Unsecured senior notes issued July 21, 2009 Minimum	Dec. 31, 2012 Due July 21, 2013, 6.10%	Sep. 30, 2012 Due July 21, 2013, 6.10%	Dec. 31, 2012 Due July 21, 2014, 6.10%	Sep. 30, 2012 Due July 21, 2014, 6.10%	Dec. 31, 2012 Due July 21, 2015, 6.10%	Sep. 30, 2012 Due July 21, 2015, 6.10%	Dec. 31, 2012 Due July 21, 2016, 6.10%	Sep. 30, 2012 Due July 21, 2016, 6.10%	Dec. 31, 2012 Unsecured revolving credit facility mature May 25, 2017	Dec. 31, 2012 Unsecured revolving credit facility mature May 25, 2017 Maximum	Dec. 31, 2012 Unsecured revolving credit facility mature May 25, 2017 Minimum	Dec. 31, 2011 Letter of credit, Replace collateral trusts item	Dec. 31, 2012 Letter of credit, Replace collateral trusts item	Dec. 31, 2012 Letter of credit, Line of credit guarantee item	Dec. 31, 2012 Letter of credit, Issued separate from unsecured credit facility item	Dec. 31, 2012 Letter of credit, Payment guarantee	Dec. 31, 2012 Letter of credit, Performance guarantee item
Debt
Debt outstanding		$ 235,000,000	$ 235,000,000	$ 75,000,000	$ 75,000,000		$ 160,000,000			$ 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		40,000,000	40,000,000
Long-term debt		195,000,000	195,000,000
Long-term debt stated interest rate percentage				6.56%			6.10%			6.10%		6.10%		6.10%		6.10%
Ratio of debt to total capitalization (as a percent)						55.00%
Annual principal repayments							40,000,000
Funded leverage ratio (as a percent)								55.00%											50.00%
Interest coverage ratio									2.50											3.00
Letters of credit outstanding/issued																							30,700,000		200,000	12,000,000
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%
Available borrowing capacity excluding letters of credit transaction																		269,300,000
Number of funded collateral trusts																					2
Proceeds from termination of collateral trusts	$ 26,100,000
Number of letters of credit outstanding																						2	3	3		2
Number of performance guaranties																										2